SOL-GEL SYNTHESIS, CRYSTAL STRUCTURE, MAGNETIC, …

SOL-GEL SYNTHESIS, CRYSTAL STRUCTURE, MAGNETIC,

ELECTRONIC AND OPTICAL PROPERTIES IN Bi2+xAxD4-3xO7+δ

(A=Al, Ce, Yb, Ga; D=Ni, Pd) NANOCOMPOSITE OXIDES

Thesis

submitted to the Pondicherry University

for the award of the degree of

DOCTOR OF PHILOSOPHY

in

CHEMISTRY

by

M. YOGAPRIYA, M. Sc.

Research Supervisor

Dr. BIDHU BHUSAN DAS

DEPARTMENT OF CHEMISTRY

PONDICHERRY UNIVERSITY

PUDUCHERRY - 605 014.

INDIA

JUNE, 2012

Materials are like People

It’s the defects that make them interesting…

Dedicated to my beloved Family, Friends &

Teachers

i

Dr. BIDHU BHUSAN DAS Professor

Department of Chemistry Pondicherry University R. Venkatraman Nagar

Kalapet Puducherry - 605 014. India

Tel: 091-413-2654413, ext. 413 (O) Email: [email protected]

_______________________________________________________________________

CERTIFICATE

This is to certify that the thesis entitled “SOL-GEL SYNTHESIS, CRYSTAL

STRUCTURE, MAGNETIC, ELECTRONIC AND OPTICAL PROPERTIES IN

Bi2+xAxD4-3xO7+δ

(A=Al, Ce, Yb, Ga; D=Ni, Pd) NANOCOMPOSITE OXIDES”

submitted to Pondicherry University, for the award of the degree of Doctor of Philosophy

is a bonafide record of research work carried out by Ms. M. Yogapriya, in the

Department of Chemistry, Pondicherry University, Puducherry - 605 014, India, under

my guidance and supervision. This is to certify that the thesis represents his independent,

original work without forming previously any part of the material for the award of any

degree, diploma or any other similar title in any University.

Puducherry (Dr. Bidhu Bhusan Das)

Date: 25.06.2012 Supervisor

mailto:[email protected]�

ii

DECLARATION

I hereby declare that the thesis entitled “SOL-GEL SYNTHESIS, CRYSTAL

STRUCTURE, MAGNETIC, ELECTRONIC AND OPTICAL PROPERTIES IN

Bi2+xAxD4-3xO7+δ

(A=Al, Ce, Yb, Ga; D=Ni, Pd) NANOCOMPOSITE OXIDES”

submitted to the Pondicherry University, Puducherry, India, in partial fulfillment of the

requirements for the award of the degree of Doctor of Philosophy is the original and

independent work carried out by me, in the Department of Chemistry, under the

supervision of Dr. Bidhu Bhusan Das, Professor, Department of Chemistry, Pondicherry

University, Puducherry. I also declare that this work, in part or full, has not formed the

basis for the award of any degree, diploma, or any other similar titles.

Puducherry (M. Yogapriya)

Date: 25.06.2012

iii

ACKNOWLEDGEMENT Many people have helped me accomplish this dissertation, and I owe my gratitude

to all of them. First and foremost I would like to express my sincere gratitude to Dr. Bidhu

Bhusan Das, Professor and Head, Department of Chemistry, Pondicherry University, without whose guidance and advice this dissertation would not be what it is today. His constant encouragement during the course of the research work and invaluable guidance with patient rescued me from despair on countless occasions. I express my heartfelt gratitude to my Doctoral Committee members Prof. Late. P. Sambasiva Rao, Department of Chemistry and Dr. S. Sivaprakasam, Department of Physics, Pondicherry University for offering their expertise throughout the period of research work. I am indebted to Dr. H. Surya Prakash Rao, Professor and Dean, School of Physical, Chemical and Applied Sciences, Dr. Late. P. Sambasiva Rao, Department of Chemistry and other faculty members Dr. K. Anbalagan, Dr. K. Tharanikkarasu, Dr. R. Venkatesan, Dr. Bala. Manimaran, Dr. G. Vasuki, Dr. K. Bakthadoss, Dr. C Sivasankar, Dr. N. Dastagiri Reddy, Dr. M.M. Balakrishnarajan and Dr. C.R. Ramanathan, Dr. Binoy Krishna Saha, Dr. S. Sabiah, Dr. Toka Swu and Dr. R. Padmanaban, Department of Chemistry, Pondicherry University for their encouragement. I especially thanks to Dr. M.M. Balakrishnarajan providing softwares to proceed the theoretical calculations. I remit my thanks to all the office staff for their help. I thank Mr. Gugan and all non-teaching staff members of the Department of Chemistry for their help. I am much obliged to Dr. M.J. Nilges, Assistant Director, Illinois EPR Research Centre, Illinois, USA, for his assistance with EPR data of the samples at 5 K. My grateful thanks to Prof. M.S. Pandian, Department of Earth Science, Prof. K. Porsezian and Prof. G. govindaraj, Department of Physics, Pondicherry University who extended the helping hands to record XRD of the samples. I wish to thank SAIF, I.I.T Madras in recording the DSC/TGA/DTA traces of samples. I wish to express my thanks to Head, Technicians and Staff members of Central

Instrumental Facility, Pondicherry University, Er. S. Ramasamy, Technical Officer, Mr. Elumalai, Mr. Gopalakrishnan, and Ms Elisa, CIF, Pondicherry University, in recording the Thermal studies, UV-vis spectra, VSM, SEM and EDX data.

iv

I thank all my lab-mates Dr. R.K. Sharma, Dr. A. Srinivassan, Mr. S. Ramesh, Mr. K. Palanisamy, Mr. Govinda Rao, Mr. Chandru, Mr. Jubin Jose, Mr. P.A.M. Ziyad, Mr. Muthuraj, Mr. Thangaraj, Mrs. K. Sumathi, Mr. D. Prabhu, Mr. Umer Rafiq, Mr. Guru doss Gupta and M.Sc students for their help.

I thank all Research Scholars in Department of Chemistry, Mr. E. Gnanamani and Mr. Suman Bhattacharya for their moral support and help and Mr. S. Boobalan, Mr. K.Velavan, Mr. K. Parthiban, Ms. P. Manochitra, Ms. S. Ramachitra and P. Sathiya from the EPR research group for their help in recording EPR Spectra. Mr. Venkat Ramaiah and Mr. Namitharan for recording DRS Spectra, Ms. P. Muthu Austeria from Chemical Information Sciences lab, Mrs. Maharaja mahalakshmi, Mr. Ganesh, Mr. Manjunathan, Mr. Thirumurugan from photochemistry laboratory Mr. K. Bakthavachalam, Ms. K. Maheswari, Mr. N.M. Rajendiran from Dr.NDR lab, Mr. S. Karthikeyan and Mr. M. Karthikeyan from Dr. B.M. lab, Mr. A. Parthiban from Dr. H.S.P. lab for their help. I would like to thank the helping hands from department of physics Mrs. R. Elillarassi and Mr. Panneer Muthuselvam for their help.

Despite all the support and help I have received, I am solely responsible for any mistakes herein.

I would like to say big thank-you to Mr. K. Balaraman for being supportive and helping me throughout my Ph. D work.

I wish to express my sincere love to all my friends Manamathi, Indhumathi, Rama, Mano, Ponni, Radhika, Austeria, Kamakshi, Kalai, Supriya, Maya, Poorni, Shalini, Revathi for their love, being supportive and patient and for giving me company while I toiled hard.

My heartfelt thank to my all teachers and well wishers for their moral support. I also wish to express my appreciation to my sister Ms. Lakshmidevi and brother

Mr. Vinothraj for their thoughtful care and concerns for me. Last, but certainly not least, I am forever grateful to my parents for showing me

the value of education. My deepest thanks go to my mom for supporting and encouraging me to achieve and be successful.

(M. Yogapriya)

v

Figure Captions

Fig. 3.1. Flowchart of the synthetic strategy of the samples in the series of Bi2+xAxD4-3x

O7+ δ

Fig. 4.1 Powder XRD indexed pattern of the system Bi

(A=Al, Ce, Yb & Ga; D=Ni & Pd) nanocomposite oxide

2+xAlxNi4-3xO7+δ

0.25, 0.50, 0.75)

(A1-A4: x=0,

Fig. 4.2.1 (a) SEM micrograph at 2 µm magnification and (b) EDX profile of Bi2Ni4O7

Fig. 4.2.2 (a) SEM micrograph at 2 and 5 µm magnifications and (b) EDX profile of

(A1) with compositions of the elements in randomly selected grain.

Bi2.25Al0.25Ni3.25O7

selected grain.

(A2) with compositions of the elements in randomly

Fig. 4.2.3 (a) SEM micrograph at 2 µm magnification and (b) EDX profile of

Bi2.5Al0.5Ni2.5O7

selected grain.


Fig. 4.2.4 (a) SEM micrograph at 2 and 1 µm magnification and (b) EDX profile of

Bi2.75Al0.75Ni1.75O7

selected grain.


Fig. 4.4.1(a) Perspective view of the unitcell of A1 (Bi8Ni16O28), (b) A2

(Bi9Al1Ni13O28

(c) A3 (Bi

)

10Al2Ni10O28), (d) A4 (Bi11Al3Ni7O28

Fig. 4.4.2 (a-d) 2-dimensional view on (001) plane of A1-A4

)

Fig. 4.4.2 (e-h) 2-dimensional view on (111) plane of Bi2+xAlxNi4-3xO7+δ

0.25, 0.50, 0.75)

(A1-A4: x=0,

Fig. 4.4.3 (a-d) Asymmetric unit of Bi2+xAlxNi4-3xO7+δ

respectively

(A1-A4: x=0, 0.25, 0.50, 0.75)

Fig. 4.4.4 (a-d) 3-dimensional electron density from (010) plane of Bi2+xAlxNi4-3xO7+δ

Fig. 4.4.5 2-dimensional electron density contour on (010) plane of Bi

(A1-A4: x=0, 0.25, 0.50, 0.75) respectively

8Ni16O28

showing the symmetric positions of Ni and O atoms on the plane.

(A1-A4)

Fig. 4.5 FT-IR spectra of samples Bi2+xAlxNi4-3xO7+δ

poly crystalline system at room temperature

(A1-A4: x=0, 0.25, 0.50, 0.75) of

Fig. 4.6.1 (a) band structure (b) density of states of Bi2Ni4O7 (A1); (c) band structure (d)

vi

density of States of Bi2.75Al0.75Ni1.75O7

Fig. 4.6.2 partial density of states of individual atoms of Bi, Al, Ni, O of

(A4)

Bi2.75Al0.75Ni1.75O7

Fig. 4.7 (a) EPR spectra at 300 K for a polycrystalline samples Bi

(A4)

2+xAlxNi4-3xO7+δ

A4: x=0, 0.25, 0.50, 0.75) respectively

(A1-

Fig. 4.7 (b) EPR spectra at 77 K for a polycrystalline samples Bi2+xAlxNi4-3xO7+δ

x=0, 0.25, 0.50, 0.75) respectively

(A1-

A4:

Fig. 4.7 (c) EPR spectra at 6 K for a polycrystalline samples Bi2.0Ni4.0O7

Bi

(A1) and

2.5Al0.5Ni2.5O7

Fig. 4.8. (a) Magnetic hysteresis loops of A1-A4 at 300 K . This show the soft ferro

(A3)

magnetic nature. (b) shows an expanded scale of the nominal composition

x=0 (A1). (c) The upper left inset shows the zoom in the low applied field

regime.

Fig. 4.9 (a) UV – vis DRS spectra of compositions Bi2+xAlxNi4-3xO7+δ

x=0, 0.25, 0.50, 0.75) respectively

(A1-A4:

Fig. 4.9 (b) Band gap values of A1-A4 obtained by plotting (αhν)2

Fig. 5.1 Powder XRD- pattern of Bi

vs. hν.

2+xCexNi4-3xO7+δ


(x = 0.25, 0.50, 0.75, 1.0) (B1-B4)

Bi2.25Ce0.25Ni3.25O7

selected grain.

(B1) with compositions of the elements in randomly

Fig. 5.2.2 (a) SEM micrograph at 10 µm magnification and (b) EDX profile of B2 with

compositions of the elements in randomly selected grain.





Fig. 5.3 TGA/DTA/DSC traces of Bi2+xCexNi4-3xO7

Fig. 5.4.1 (a-d) Perspective view of the unitcell of B1-B4

(x = 0.25, 0.50, 0.75, 1.0) (B1-B4)

Fig. 5.4.2 (a) 001 plane; (b) 111 plane; (c) asymmetric unit of B1

Fig. 5.4.3 (a) 001 plane; (b) 111 plane of B2

vii



Fig. 5.4.6 3-dimensional electron density from (010) plane of (c) B3; (d) B4

Fig. 5.4.7 2-dimensional electron density contour on (010) plane of (a-d) B1-B4


Fig. 5.5.1 (a) band structure (b) density of states of B1; (c) band structure (d) density of

states of B2

Fig. 5.5.1 (e) Band structure (f) density of states of B3; (g) band structure (h) density of

states of B4

Fig. 5.5.2 Partial density of states of individual atoms Bi, Ce, Ni and O of B4

Fig. 5.6 (a) EPR spectra of Bi2+xCexNi4-3xO7+δ

Fig. 5.6 (b) EPR spectra of Bi

(x= 0.25, 0.50, 0.75, 1.0) (B1-B4) at 300

K

2+xCexNi4-3xO7+δ

Fig. 5.7 M vs H plot showing weak hysteresis loop for B1-B4

(x = 0.25, 0.50, 0.75, 1.0) (B1-B4) at 77

K

Fig. 5.8 (a) Optical absorption spectra of Bi2+xCexNi4-3xO7+δ

(B1- B4)

(x = 0.25, 0.50, 0.75, 1.0)

Fig 5.8 (b) Variation of (αhν)2

Calculated extrapolating the curve.

as a function of energy (hν) of B1-B4. Band gap energy is

Fig. 6.1 powder X-ray diffraction pattern of C1-C4

Fig. 6.2 (a-d) SEM micrographs of C1-C4

Fig.6.2.2 (a-b) EDX profile of C1 and C2

Fig.6.2.2 (c-d) EDX profile of C3 and C4

Fig. 6.3 TGA/DTA/DSC traces of C1-C4

Fig. 6.4.1 (a-d) Perspective view of unitcell structure of C1-C4

Fig. 6.4.2 (a-d) 2-dimensional view on (001) plane of C1-C4

Fig. 6.4.3 (a-d) Asymmetric unit of C1-C4

Fig. 6.4.4 (a-d) 3-dimensional electron density from (010) plane of C1-C4

Fig. 6.4.5 2-dimensional electron density contour on (010) plane of C1-C4 showing the

symmetric positions of Bi, Pd and O atoms on the plane.

Fig. 6.5.1 (a) band structure (b) density of states of C1; (c) band structure (d) density of

viii

states of C2

Fig. 6.5.1 (e) band structure (f) density of states of C3; (g) band structure (h) density of

states C4

Fig. 6.5.2 Partial density of states of individual atoms of Bi, Pd, Yb and O of C1 and C4

Fig. 6.6 (a) EPR spectra at 300 K of C1-C4

Fig. 6.6 (b) EPR spectra at 77 K of C1-C4

Fig. 6.7.1 Magnetic hysteresis loops of C1-C4 at 300 K

Fig. 6.8 (a) Observed optical absorption spectra of C1-C4

Fig. 6.8 (b) Band gap energy calculation by plotting (αhν)2

Fig. 7.1 Powder XRD patterns of Bi

vs. hν

2+xGaxPd4-3xO7+δ

Fig.7.2 (a-d) SEM micrographs of Bi

(D1-D4: x=0.15, 0.30, 0.45 & 0.60)

2+xGaxPd4-3xO7+δ

at different magnifications

(D1-D4: x=0.15, 0.30, 0.45 &

0.60)

Fig.7.3. TGA/DTA/DSC traces of Bi2+xGaxPd4-3xO7+δ

Fig. 7.4.1 Perspective view of the unitcell of Bi

(D1-D4: x=0.15, 0.30, 0.45, 0.60)

2+xGaxPd4-3xO7+δ

0.45 & 0.60)

(D1-D4: x=0.15, 0.30,

Fig. 7.4.2 2-dimensional view on (001) plane of Bi2+xGaxPd4-3xO7+δ

0.30, 0.45 & 0.60) respectively

(D1-D4: x=0.15,

Fig. 7.4.3 (a-d) Asymmetric unit of Bi2+xGaxPd4-3xO7+δ

0.60) respectively

(D1-D4: x=0.15, 0.30, 0.45 &

Fig. 7.4.4 (a-d) 3-dimensional electron density from (001) plane of Bi2+xGaxPd4-3xO7+δ

(D1-D4: x=0.15, 0.30, 0.45 & 0.60)

Fig. 7.4.5 (a-d) 2-dimensional electron density contour on (010) plane of Bi2+xGaxPd4-3x

O7+δ

Fig. 7.5.1 (a) band structure (b) density of states of D1; (c) band structure (d) density of

(D1-D4: x=0.15, 0.30, 0.45 & 0.60) respectively

States of D2

Fig. 7.5.1 (e) band structure (f) density of states of D3; (g) band structure (h) density of

States of D4

Fig. 7.5.2 Partial Density of states of Bi, O, Ga, Pd in D4

Fig. 7.6.1 EPR spectra at 300 K of D1-D4 respectively


ix

Fig. 7.6.3 EPR spectra shows the zoom in the field around 3000 G at 77 K of D1

and D2 respectively

Fig. 7.7 Magnetic hysteresis loops of D1-D4 at 300 K

Fig. 7.8 (a) Observed optical absorption spectra of D1-D4

Fig. 7.8 (b) Band gap energy of D1-D4 obtained by plotting (αhν)2

vs. hν

List of Tables

Table 3.1 Compositions of samples in the series of Bi2+xAxD4-3xO7+ δ

D=Ni, Pd) nanocomposite oxide

(A=Al, Ce, Yb, Ga;

Table 4.1 Unit cell parameters, calculated and observed densities and average crystallite

sizes in A1-A4 of Bi2+xAlxNi4-3xO7+δ

Table 4.4.1 Unit cell dimension, a (Å), and the agreement factors after Rietveld

(0.0 ≤ x ≤ 0.75) composite oxides

refinement of the unit cell structure of A1-A4 of Bi2+xAlxNi4-3xO7+δ

0.75) composite oxides.

(0.0 ≤x≤

Table 4.4.2 Selected Bond lengths and bond angles of Bi2+xAlxNi4-3xO7+δ

system

(0.0 ≤ x ≤ 0.75)

Table 4.4.3 Generated positions of atoms in the asymmetric unit of Bi2+xAlxNi4-3xO7+δ

(0.0 ≤ x ≤ 0.75) system

Table 4.7 Observed concentration of Ni2+ ions and observed EPR giso

300 K, 77 K and 6 K of Bi

-values of A1-A4 at

2+xAlxNi4-3xO7+δ

system.

(0.0 ≤ x ≤ 0.75) composite oxide

Table 4.8 Observed magnetization (Ms), coercivity (Hci), retentivity (Mr

the hysteresis loop, magnetic susceptibility(χ), Weiss constant(θ) and

), total area of

exchange integral (j) of A1-A4 of Bi2+xAlxNi4-3xO7+δ

Table 4.9. Direct band gap values of A1-A4 calculated using absorption coefficient.

(0.0 ≤ x ≤ 0.75) system.


sizes in B1-B4 of Bi2+xCexNi4-3xO7+δ

Table 5.2 Generated Cartesian coordinates (xyz) before and after refinement of the


atomic positions in the asymmetric unit in B1-B4.

Table 5.3 (a) Selected bond length of B1

x

Table 5.3 (b) Selected bond length of B2

Table 5.3 (c) Selected bond length of B3

Table 5.3 (d) Selected bond length of B4

Table 5.4 (a) Selected bond angle of B1

Table 5.4 (b) Selected bond angle of B2

Table 5.4 (c) Selected bond angle of B3

Table 5.4 (d) selected bond angle of B4

Table 5.7. Observed magnetization (Ms), coercivity (Hci), retentivity (Mr

the hysteresis and magnetic susceptibility, of Bi

), total area of

2+xCexNi4-3xO7+δ

≤1.25) system.

(0.25 ≤ x

Table 5.8 Band gap energy values of B1-B4 respectively


sizes in C1-C4 of Bi2+xYbxPd4-3xO7+δ

Table 6.4.1 Generated positions of atoms in the asymmetric unit


Table 6.4.2 Bond angles between the atoms

Table 6.4.3 (a) Bond length of C1

Table 6.4.3 (b) Bond length of C2

Table 6.4.3 (c) Bond length of C3

Table 6.4.3(d) Bond length of C4

Table 6.6 Observed g – values in C1-C4



), total area of

2+xYbxPd4-3xO7+δ

0.75) system.

(0.0 ≤ x ≤

Table 6.8. Direct band gap energy values of C1-C4


sizes in D1-D4 of Bi2+xGaxPd4-3xO7+δ

Table 7.4.1 Generated positions of atoms in the asymmetric unit


Table 7.4.2 Reliability factors

Table 7.4.3 Selected bond angles for D1-D4

Table 7.4.4 Selected bond lengths of D1-D4

Table 7.6 Observed g - values in D1-D4

xi


the hysteresis, magnetic susceptibility, weiss constant and exchange integral

), total area of

of Bi2+xGaxPd4-3xO7+δ

Table 7.8 Direct band gap energy values of D1-D4

(0.15 ≤ x ≤ 0.60) system

SYNOPSIS

The systematic studies and correlating the properties with the structures of solid

materials has been the subject of intense investigations due to their application

potentialities in many areas. The transition metal nanocomposite oxides which normally

are relatively easy to prepare with low cost by sol-gel method as well as the interesting

diverse physical properties have attracted much attention as the new materials in recent

years. Tailoring specific properties of solids such as magnetic, optical, electronic etc. is

important to utilize the solid as a material. However, it is necessary to ensure if the

material is monophasic and to what extent for such applications.

Currently the increasing interest in designing and preparing rare-earth - transition

metal oxides have also received extensive attention due to their unique 3d-4f orbital

interactions and fascinating chemical and physical properties such as electronic energy

band structures and magnetic ordering that results from two different types of orbitals

with distinct energy levels (3d of transition metal and 4f of rare-earth ions). In these

materials the 4f electrons of the rare earth ions polarize their 5d bands, which give the 5d-

3d short range interaction with the transition metal. The 5d band polarizations are

oriented parallel to 4f moments because the local 4f-5d exchange interactions are

positive.

xii

The study of physical properties of such materials requires single-phase samples

as the electronic behaviour and magnetic properties are closely related to the structure.

Therefore, it is necessary to establish whether the samples are indeed monophasic and to

what extent. The sol-gel method, a versatile and relatively low-cost technique makes it

possible the prepare varieties of solid materials, crystalline as well as amorphous, whose

structure and other properties differ significantly from that of solid materials prepared by

other traditional solid state techniques.

In this thesis, series of monophasic nanocomposite oxides in the series of

Bi2+xAxD4-3xO7+δ

Chapter - 1:

(A=Al, Ce, Yb & Ga; D=Ni & Pd) are synthesized by sol-gel method

via nitrate-citrate route and their magnetic, optical and electronic properties are studied

by a varieties of direct structure-sensitive techniques.

This chapter comprises an adequately thorough literature survey on

nanocomposite metal oxides with special emphasis on transition metal and rare-earth

mixed oxides and their chemical and physical properties such as crystal structures,

structural morphology and stability, and potential applications as magnetic, optical and

electronic materials with relevant references. A short introduction to the applications of

nanostructured metal oxides is also included. Importance of transition metal-rare-earth

metal based single phase materials has been discussed at length in this chapter. The main

objectives and the scope of the present investigation are also outlined as clearly as

possible.

Chapter - 2:

xiii

In this chapter, the experimental procedures and the techniques employed in this

investigation are descried with necessary theoretical background.

Chapter – 3:

Chapter - 3 describes the complete synthesis of the samples using sol-gel method

via nitrate-citrate precursor route and characterization techniques of the samples studied

are described. In addition to the preparation procedures, the characterization techniques

include powder x-ray diffraction (XRD), differential scanning calorimetry (DSC),

differential thermal analysis (DTA), thermogravimetry (TG), scanning electron

microscopy (SEM), energy dispersive analysis of x-rays (EDX), magnetic measurements

by vibrating sample magnetometer (VSM), infra-red (IR) spectroscopy, electron

magnetic resonance (EMR) spectroscopy in the range 5-300 K, optical absorption studies,

chemical analysis, density measurements and calculations of electronic properties such as

energy band structures, density of states (DOS) and related optical properties using

CASTEP (Cambridge Serial Total Energy Package) programme package which uses the

plane-wave density functional theory (DFT).

Chapter - 4:

Chapter - 4 describes the results and discussion of Bi2+xAlxNi4-3xO7+δ (A1-A4: x

= 0.0, 0.25, 0.50, 0.75) (system-A) nanocomposite oxides. Analysis of the powder XRD

patterns by Fullprof shows cubic unit cell with lattice parameters in A1-A4: a = 10.2678,

10.1197, 10.1183, 10.1134 Å and space group Pm3n. From the comparison of the

observed (A1-A4: 4.398, 4.946, 5.169, 4.825 g/cm3) and calculated (A1-A4: 4.680,

4.999, 5.097, 4.520 g/cm3) densities, the Z value is determined to be 4. The average

crystallite sizes in A1-A4 determined by Scherrer’s relation are found to be in the range ~

xiv

42-61 nm. The DTA-TG results show no phase transitions in the range 25-800 ºC. SEM

micrographs show that as the concentration of aluminuim increases the shape of the

particles changes from globular shape to quadrupods-like shape in A1 to A4. Rietveld

refinement of the unit cell structure developed on space group Pm3n shows appreciably

lowered agreement factors Rp = 97.10, 96.19, 96.14, 95.97 %; Rwp = 99.83, 96.80, 96.98,

96.37 %; Rexp = 0.23, 0.26, 0.30, 0.23 % in A1-A4, respectively. The Bi-O and Ni-O

bonds of different sites show marginal variations in the samples. Circular nature of the

Fourier electron density 2D-contours around Ni and O due to valence orbitals shows the

partial ionic character of Ni-O bonds. The IR spectral results at 300 K show Ni-O (~ 411

cm-1), Bi-O (~441 cm-1) and Bi-O (471 cm-1) in [BiO6/2] octahedral units. Diffuse

reflectance spectra (DRS) at 300 K show absorption at 420 nm due to 3A2g (F) →3A2g

(P), 460 nm due to 3T1g (F) → 1T2g (D), 640 nm due to 3A2g (F) → 3T1g (F), and at 720

nm due to 3A2g (F) – 3T1g (P) transitions. The DFT calculations on unit cell structure of

A1-A4 are done for electronic energy band structures and density of states (DOS) using a

CASTEP programme package. This result shows the transition of semiconductor-like as

in A1 to metallic character with the Ni eg band predominantly crossing the Fermi level, in

A2-A4. The presence of hysteresis loop with very low values of the coercivity,

retentivity, magnetic susceptibility ~10-5 emu/gG with saturation magnetic field ~ 4 kG

shows fairly weak ferromagnetic nature of the samples at 300 K. The observed X-band

EMR lineshapes at 300, 77 and 6 K are isotropic in nature. The similar giso-values in A1-

A4 in the range 300-6 K show the similar local structure and the paramagnetic site

symmetry in the samples. The lineshapes at 300 K show decreasing intensity from A1 to

A4 due to the dilution of the magnetic ions by addition of more diamagnetic Bi3+ ions and

xv

substitution of Ni2+ (3d8, S=1, in strong octahdral field) by Al3+ ions. The very weak 4-

line feature in A1-A4 is due to the Ni+ (58Ni, I= 0; 61Ni, I=3/2, natural abundance

~1.25%) ions. The mI-dependent EMR linewidths due to Ni+ ions broaden due to the

shorter relaxation time at 77 K. The broad isotropic lineshape at 6 K is also

predominantly due to the Ni+

ions.

Chapter – 5:

Chapter - 5 describes the results and discussion of Bi2+xCexNi4-3xO7+δ (x = 0.25,

0.50, 0.75, 1.0) (B1-B4) (system-B) nanocomposite oxides and correlating the results

with that of Chapter- 4. Instead of non-magnetic aluminium ions as in system- A,

magnetic rare-earth cerium Ce3+ (4f1) ions are substituted and the variations/similarities in

properties are discussed. Powder-XRD patterns were analyzed and the results show the

cubic phase in all the samples and the lattice parameters are a = 10.7969, 10.8036,

10.7946 and 1.8182 Å, in B1-B4 respectively, Z=4 and space group Pa-3. The observed

values of the densities are 5.364, 4.883, 4.757, 4.854 g/cc in B1-B4, respectively. The

average crystallite sizes of the samples B1-B4 are found to be in the range 32-51 nm.

Thermogravimetric (TG) results in the range 50-1000 ºC indicated that the weight loss up

to 400 ºC is due to gradual removal of moisture present in the samples and the weight

loss around 600 ºC with an endothermic peak in DSC/DTA traces is due to reduction of

Ni2+ to Ni+ with consequent removal of O2. The SEM micrographs show the nano-rods

present in the samples. The EDX mapping of the samples show the uniform distribution

and the purity of the samples. The unit cell structure of the samples are developed with

space group Pa-3, Z = 4 with Wycoff positions, Bi3+ (1) in 8(c), Ni2+ (1) in 8(c), Ni2+ (2)

xvi

in 8(c), O2-(1) in 24(d) and O2-(2) in 4(a). The Rietveld refinement on the unit cell

structures show agreement factors (R-values) Rp = 98.57 %, Rwp = 96.55 %, Rexp = 0.18

% for the representative sample B1. Calculated bond lengths Bi1-O1: 2.3923 Å, Ni1-

O1: 2.5864 Å, Ni2-O1: 2.7506 Å show that two different Ni ions present in all the

samples. Circular nature of Fourier electron density contours shows the partial ionic

nature of Ni-O bonds in the samples. The FT-IR spectroscopic results show the presence

of Ni-O, Bi-O and Ce-O bonds in the samples. The magnetic moment versus magnetic

field plots show hysteresis behavior with low values of coercivity and retentivity

indicating the weak ferromagnetic nature of the samples as observed in the case of A1-

A4. The optical absorption studies show the 3A2g →3T1g (350 -380 nm), 3A2g → 1A1g

(415 nm), 3A2g →1T1g (460 nm), 2F5/2 → 2T2g (4f1→5d1)) (280-350), 3A2g (F) → 3T1g (P)

(720 nm) transitions in the samples. The DFT calculations of electronic and optical

properties of the samples have been reported using a plane-wave method as implemented

code in CASTEP. The band structure and density of states show the metallic character

with the Ni 3d band predominantly crossing the Fermi level and with no high-energy Bi

or Ce band present. The energy band gap, Eg, is found to be ~ 0.01 eV in B1-B4. The

observed X-band EMR lineshapes at 300 K are isotropic except in B2. The EMR spectra

show the presence of two g-values for B2 (x =0.5) g║ = 2.429, g┴ = 2.081 and g = 2.003,

due to formation of two different Ni2+ (58Ni, I=0, NA ~97%; 61Ni, I=3/2, ~1.37%) sites

and giso = 2.065, 2.003, 2.002 for B1, B3 and B4 (x =0.25, 0.75 and 1.0) respectively.

The EMR lineshapes at 77 K also show two isotropic g-values g1= 2.852, 2.836, 2.836,

2.836 which correspond to Ce3+(4f1) in B1-B4 and poorly resolved 6-line feature and g2

= 2.018, 2.009, 2.009, 2.026 with poorly resolved 4-line feature due to Ni2+ ions in B1-

xvii

B4, respectively. The marginal decrease in g-values is due to the addition of diamagnetic

Bi3+ (6s2

Chapter - 6:

) ions in the matrix in place of Ni ions which partially distorts the site symmetry

of the Ni ions in B1-B4.

The Chapter - 6 describes the results and discussion of the Bi2+xYbxPd4-3xO7+δ (x

= 0.0, 0.25, 0.50 and 0.75) (System-C) (C1-C4) nanocomposites. Analysis of the powder

XRD patterns by Fullprof shows tetragonal unit cell with lattice parameters in C1-C4: a =

8.614, 8.615, 8.615, 10.904 Å; c = 5.911, 5.910, 5.906, 7.113 Å and space group

P4/NCC. The observed density values of C1-C4 are 2.9775, 3.4306, 3.7685, 4.2652 g/cc,

respectively. These values are compared with the calculated ones (C1-C4: 3.6200,

3.6787, 3.7400, 4.2962 g/cm3) and the Z value is found out to be 1. The DTA/DSC/TGA

traces in the range 30- 1000 °C shows that the samples are stable in temperature up to

940 °C. SEM micrographs show similar globular nature for all samples but the size of

the crystallites decreases when the x-value (concentration of Pb3+ ions) increases from

0.0 to 0.75. Rietveld refinement of the unit cell structure developed on space group

P4/NCC, Z=1 with Wycoff sites: Bi3+ in 4(c), Pd2+ in 4(a), O2- in 8(f) in the samples

C1-C4 is shows the agreement factors: Rp= 94.14 %, Rwp= 94.96 % and Rexp=0.17 % for

the representative sample C1. FT-IR spectra show the band 550 cm-1 corresponds to Pd-O

stretching vibration in samples C1-C4. The optical absorption spectra show band in the

range ~ 224-283 nm due to the O2- → Pd2+ ( 4d8) charge transfer and the wide band in the

range 400-500 nm is attributed to Pd2+ d-d transitions in distorted octahedral

coordination. The electronic energy band structure and density of states in the samples

are calculated DFT by using CASTEP code. The band gaps in the samples ~ 0.007-0.008

eV in C1-C4 show fairly metallic nature in the samples. The band in the range ~ -5.0-1.8

xviii

eV, is attributed to the overlap of Pd-4d and Bi 6p bands with O-2p bands. This also

indicates well-defined Pd-O, Bi-O and Pd-O-Bi co-ordinations in all the samples, and this

result is in conformity with the bond lengths and bond angles calculated using powder

XRD results described earlier. The presence of indistinct hysteresis loops in the magnetic

moment versus magnetic field plots indicates possible weak ferromagnetic nature in the

samples. Since the shape of a hysteresis loop is determined partly by the domain state

such behavior is attributed to the widely distributed domain sizes in the samples. The

observed EMR lineshapes at 300 K show g-values: g║ = 2.833, 2.833, 2.874, 2.833; g⊥ =

2.085, 2.053, 2.086, 2.108 due to Pd+ (4d9) (105Pd, I=5/2, abundance ~ 22.2%) ions in

axially distorted octahedral coordination. At 77 K the observed g-values are g1 = 3.509,

3.434, 3.437, 3.442 due to Yb3+ ions (171Yb, 4f13, S=1/2, I=1/2) and g2 = 2.388, 2.409,

2.411, 2.415 due to Pd2+

Chapter - 7:

ions in distorted octahedral coordination in C1-C4, respectively.

The Chapter -7 describes the results and discussions of Bi2+xGaxPd4-3xO7+δ (D1-

D4: x=0.15, 0.30, 0.45, 0.60) (System-D) where the diamagnetic Ga3+ (3d10) in is

incorporated in place of Yb3+(4f13) as in Chapter-6. Powder XRD results analyzed by

Fullprof show tetragonal phase with lattice parameter a = 8.6157, 8.6267, 8.6228 and

8.6256 Å; c = 5.9183, 5.9274, 5.9104 and 5.9148 Å, space group I4/mcm and Z = 1. The

observed density values are D1-D4: 3.365, 3.423, 3.425, 2.892 g/cc. TG-DTA traces

show weight loss up to 400 ⁰C due the loss of water as observed in the case of B1-B4.

DTA/DSC traces show an endothermic peak at ~ 600 ⁰C which corresponds to the

reduction of Pd2+ to Pd+ ions in all the samples. SEM micrographs show fairly similar

morphology of the samples D1-D4 with distinct rod-shaped particles in D4(x=0.60). The

xix

structure of the unit cells are developed using the space group I4/MCM and Z=1 with

Wycoff sites: Bi3+ in 4(c), Pd2+ in 4(b), and O2- in 8(c). Rietveld refinement of the unit

cell structure show appreciably lowered agreement factors Rp = 94.88, 93.78, 95.55,

93.43 %; Rwp = 95.81, 95.88, 96.63, 94.45 % and Rexp = 0.22, 0.91, 0.20, 0.26 % in D1-

D4, respectively. The calculated bond lengths, bond angles show only marginal variations

in the samples. FT-IR spectra show the band 550 cm-1 which corresponds to Pd-O

stretching in samples D1-D4. Optical absorption band between 400-500 nm is assigned to

the d–d transitions in Pd2+ ions. The calculated band structure, density of states and

optical properties in the samples are done using CASTEP programme package. The band

gap in the samples is found to be ~ 0.015 eV in D1-D4, respectively. The band gap

increases by ~0.007 eV in D1-D4 as compared with those in C1-C4, because of the

substitutional effect of Ga3+ ions in D1-D4. This result further shows that the samples

D1-D4 are tend towards semiconductor-like with respect to C1-C4. The presence of

hysteresis loops with very low values of the coercivity, retentivity, magnetic

susceptibility ~10-5 emu/gG shows fairly weak ferromagnetic nature of the samples at

300 K. However, unlike the samples C1-C4, the magnetization and coercivity values

decrease from D1(x = 0.15) to D4(x = 0.60) because the magnetic Pd2+(4d8) ions are

substituted by non-magnetic Ga3+(3d10) ions. The observed EPR lineshapes at 300 K

show two g-values: g1= 2.024, 2.018, 2.010, 2.033; g2= 4.260, 4.201, 4.184, 4.186 at 300

K and also two g-values: g3 = 2.148, 2.148, 2.041, 2.051; g4 = 4.423, 4.186, 4.209, 4.204

at 77 K in D1-D4, respectively. The g-values ~2.0 is due to the Pd+ (4d8) (105Pd, I=5/2,

abundance ~ 22.2%) and Pd2+(4d9) (105Pd, I=5/2, abundance ~ 22.2%) sites in D1-D4.

xx

While the g-values ~ 4.0 is attributed to the presence of magnetic impurity ions in the

samples.

Chapter - 8:

The thesis concludes with Chapter-8 with highlights of the overall findings in our

study, and with proposed prospects for further studies in such series. The new series of

Bi2+xAxD4-3xO7+δ (A=Al, Ce, Yb & Ga; D=Ni & Pd) nanocomposite oxides have been

prepared via sol-gel method and their structure-property relations have been studied by

various solid state techniques. Powder XRD results show single in all the systems.

System-A show cubic unit cell with the space group Pm3n and Z = 4; system-B shows

cubic unit cell with space group Pa-3 and Z = 4; system-C shows tetragonal unit cell with

P4/NCC and Z = 1; and system-D shows tetragonal unit cell with space group P42/NCM

and Z = 1. This result also shows that the same compositional formula does not imply the

same phase in the samples. The average crystallite sizes calculated using Scherrer’s

relation are in nanometer range for all the samples in the four systems studied.

DTA/DSC/TGA traces show the similar hygroscopic behavior as well as the thermal

stability of the samples in the various systems. The SEM micrographs show the similar

morphology in the samples in a system but marginal variation with the different systems.

The EDAX results show the stoichiometry close to nominal in the samples. Structure of

the unit cells are developed using the corresponding space group and Rietveld refinement

has done on the structure. Fourier electron density mapping convinces the structural

features. FT-IR spectral results show the presence of Ni-O and Bi-O bonds in system-A,

Ni-O, Bi-O and Ce-O bonds in system-B, Pd-O bond in system-C and system-D in

conformity with the powder XRD results.

xxi

The calculated electronic energy band structure, density of states and optical

properties were done using the commercial version of the software package CASTEP

code. The results show semiconductor-like behavior in A1 to metallic character in A2-

A4; semiconductor-like in B1-B4; fairly metallic nature in C1-C4; and semiconductor-

like in D1-D4. These results help us to infer that in such series synthesis of monophasic

nanocomposite oxides are quite possible for exploitation of their various physical

properties as well as their applications as semiconductors or composite metals. The

samples in all the systems exhibit hysteresis loops compatible with weak ferromagnetic

nature of the samples at 300 K. The EPR results in the range 6-300 K in system-A, and at

77 k and at 300 K in systems-B, -C and -D show the oxidation states of the magnetic ions

and cationic distribution in the samples.

The magnetic anisotropy and coercive fields are important properties of soft

magnetic materials which find applications in the magnetic recording and reading

technologies in modern devices. These modern devices exclusively need soft magnetic

materials as its basic magnetic component. Soft magnetic materials are extensively used

in inductors which form a basic requirement in high technology areas and the above

studied Bi2+xAxD4-3xO7+δ

(A=Al, Ce, Yb & Ga; D=Ni & Pd) nanocomposite oxides

series adequately suit these demands and are considered to shape the future of advanced

technology.

xxii

CONTENTS Page

No.

Certificate … i

Declaration … ii

Acknowledgement … iii

Figure Captions … v

List of Tables … ix

Synopsis … xi

Contents …xxii

Chapter I Introduction … 01

1.1 Introduction to solid state materials chemistry … 01

1.2 Transition metal oxide materials … 01

1.3 Rare-earth - transition metal composite oxide materials … 02

1.4 4d-Transition metal oxides … 03

1.5 Group III-A metal ion dopants … 04

xxiii

1.6 Novel synthetic strategy … 04

1.7 Applications of nanocomposite oxide materials … 06

1.7.1 Soft Magnetic materials … 06

1.7.2 Nanotechnology … 06

1.7.3 Soft Magnetic materials … 07

1.8 Theoretical developments … 07

1.9 Present work … 08

References … 10

Chapter II Theoretical background … 12

2.1 X-ray Diffraction Studies … 12

2.2 Crystallite Size Determination … 13

2.3 Scanning Electron Microscopy … 13

2.4 Energy dispersive X-ray Analysis (EDX) … 14

2.5 Thermal Studies … 14

2.6 UV-vis Diffuse Reflectance Spectroscopy … 15

2.7 Theory of magnetism … 16

2.8 DFT Calculations … 18

References … 24

Chapter III Experimentals … 25

3.1 Sample Preparation … 25

3.2 X-ray Diffraction studies … 28

3.3 Scanning Electron Microscopy and

Energy Dispersive X-rays … 28

xxiv

3.4 Thermal Studies … 28

3.5 FT-IR Spectroscopy … 29

3.6 EPR Spectroscopy … 29

3.7 Magnetic Susceptibility measurements … 30

3.8 Optical Absorption studies … 31

3.9 Determination of Density … 31

3.10 Band structure and density of states calculations … 32

References … 33

Chapter IV Structure-Property Relations in

New Bi2+xAlxNi4-3xO7+ δ

4.1 Introduction … 34

(0.0 ≤ x ≤ 0.75) Series

… 34

4.2 Results and Discussion … 35

4.2.1 Powder X-ray diffraction Studies … 35

4.2.2 Microstructure and EDX … 37

4.2.3 Thermal Studies … 38

4.2.4 Crystal Structure Refinement and

Electron Density Fourier Mapping … 38

4.2.5 FT-IR Studies … 43

4.2.6 Band Structure and Density of States … 53

4.2.7 Electron Paramagnetic Resonance Spectral Studies … 57

4.2.8 Magnetic Studies … 63

xxv

4.2.9 Optical Absorption Studies … 65

4.3 Conclusion … 68

References … 70

Chapter V Structure-Property Relations in

New Bi2+xCexNi4-3xO7+δ


(0.25 ≤ x ≤ 1.0) Series … 72



5.2.2 SEM and EDX … 75









References … 105

Chapter VI Structure-Property Relations in

New Bi2+xYbxPd4-3xO7+δ


(x = 0.0, 0.25, 0.50 and 0.75) … 107



6.2.2 SEM and EDX … 108

xxvi









References … 136

Chapter VII Structure-Property Relations in

New Bi2+xGaxPd4-3xO7+δ


(x = 0.15, 0.30, 0.45 and 0.60) Series …

138

6.2 Results and Discussions … 139


6.2.2 SEM Micrographs … 141









xxvii

References … 165

Chapter VIII Summary, Conclusions and Future work … 166

1

Chapter – I

Introduction

1.1 Introduction to solid state materials chemistry

Solid state materials chemistry has become a distinct discipline of chemical

sciences for the past two decades. This area now encompasses an interface between

chemistry, physics, engineering and biological sciences. The spirit of research in

chemistry is the art in synthesis, and it is also appropriate for materials chemistry. If new

compounds or materials are not synthesized, then the discovery of newer properties in

materials also cannot be found. It is one of those imminent challenges of modern

materials chemistry to develop novel materials that have comparable or better

functionalities1 than those presently used. The systematic studies and correlating the

properties with the structures of solid materials is focused primarily due to their

application potentialities in many areas2

1.2 Transition metal oxide materials

.

The design and synthesis of transition metal oxide materials provide a variety of

applications of electronic3 and magnetic devices4, heterogeneous catalysis5

Transition metal oxides play a vital role in several areas of chemistry, physics,

materials science etc. The unusual properties of transition metal oxides are clearly due to

the unique nature of the valence d-electrons. The affluent diversity of compositions,

and a number

of other applications. Although design and synthesis of materials is one of the challenges

in materials chemistry, it is far from achieving this goal universally. Synthesis of

materials can be achieved but the success has been limited. To accomplish the target

properties, the right structure and composition have to be achieved.

2

structure-properties relations of metal oxides has led to extensive applications in

electronics, engineering, and catalysis. By using a variety of synthetic techniques6, one

can justifiably induce a large diversity of properties in the oxide materials. The desire for

novel functions or properties generate a vast demand for new materials which has to be

fabricated with well defined, controllable properties and structures on the nanometer

scale afforded by new nanocomposite oxide materials. This has generated mostly the

special interest in oxide materials in tailoring the properties on the molecular and

nanoscale level, generating novel materials that either shows characteristics in between

the original phases or even new properties. Tailoring specific properties of solids such as

magnetic, optical, electronic etc. is important to utilize the solid as a material. However,

it is necessary to ensure if the material is monophasic and to what extent for such

applications7. The phenomenal range of electronic and magnetic properties exhibited by

transition metal oxides is also noteworthy8,9

1.3 Rare-earth - transition metal composite oxide materials

.

Apart from the oxide materials containing only the transition metals as mentioned

above, currently the increasing interest in designing and preparing rare-earth - transition

metal composite oxides have also received extensive attention due to their unique 3d-4f

orbital interactions. This 3d-4f orbital interactions results in fascinating chemical and

physical properties such as electronic energy band structures and magnetic ordering that

results from two different types of orbitals with distinct energy levels (3d of transition

metal and 4f of rare-earth ions). In these materials the 4f electrons of the rare earth ions

polarize their 5d bands, which give the 5d-3d short range interaction with the transition

3

metal. The 5d band polarizations are oriented parallel to 4f moments because the local

4f-5d exchange interactions are positive10

1.4 4d-Transition metal oxides

.

Recently, 4d transition-metal oxides also have attractive attention, because of

numerous intriguing properties. The 4d transition metal oxides are characterized by more

extended orbitals than those of 3d transition metal oxides. So, 4d orbitals have rather

weak on-site Coulombic repulsion energy and exchange energy, j. It can be achieved that

the strong hybridization with neighboring orbitals, e.g., O 2p orbitals, than 3d orbitals.

However, these phenomena are insufficient to understand the intriguing physical

properties observed in some 4d transition metal oxides. Except ruthenates11

Taniyama et al.

there have

been quite a few quantitative studies about the electronic structures of 4d transition metal

oxides. Perspective view of quantitative studies associated to the electronic structures of

4d transition metal oxides will offer starting point in investigating various 4d transition

metal oxides with a potential to discover other new interesting phenomena. Not only the

quantitative studies but also the comparisons with the 3d transition metal oxides cases

which provide a gateway to understand the phenomena.

12 observed experimentally the ferromagnetic behavior of

palladium clusters in 1997. The nanoscale palladium ions (4d transitions metal ions)

have novel magnetic properties which is recorded by recent reports13,14. The magnetic

properties of nanoscale Palladium are different from those of other 4d metals such as Rh

and Ru which has been predicted theoretically. The reduced coordination number, high

symmetry of atomic structure, and electron localization induced by the lattice expansion

are some of the key factors determining. The above key factors result in degenerate

4

electronic states and perhaps enhancement of the local density of states (LDOS) at the

Fermi energy level.

1.5 Group III-A metal ion dopants

Group III-A metal ion dopants are sensitive which influence the variation in band

structure by the first-principles DFT calculations. In the band structure, the donor level

above the original valence band (VB) and the acceptor level below the original

conduction band (CB), is mainly dominated by the dopants15-18

1.6 Novel synthetic strategy

. Introducing the dopants

induces the variation in the band structure and narrows the band gap. The band of the

semiconductors can be modulated to a certain degree by selectively doping with different

metals as metal ions in the same group, but with different ionic radii and

electronegativities, have different roles.

The development of novel strategies for synthetic procedure, mainly those

involving soft chemical routes is noteworthy19. The methods of soft chemistry include

sol-gel, electrochemical, hydrothermal, intercalation and ion exchange processes etc. are

employed routinely for the synthesis of composite materials. Traditionally solid state

materials are prepared by ceramic technique - by using the high temperature solid state

reactions20. Exploratory synthesis based on trial and error approach has yielded

numerous discoveries and continues to be a valuable and popular method in the chemistry

of materials. Directed synthesis is generally employed to target a particular property.

Trial and error methods and the directed synthesis approaches have provided reasonable

advances in the area of materials science. Combinatorial, high throughput and parallel

5

methods of syntheses allow for optimising experimental conditions and hold considerable

promise.

In general, for the synthesis of a solid, in addition to the high temperature

approach, synthesis from fluxes and melts, hydrothermal methods and also preparation

from solutions have been employed. Recently, synthesis of extended framework

compounds by directed assembly of molecular building blocks, low-temperature

synthesis of oxides and chalcogenides in the nanometer sizes, are emerging as important

developments. In spite of the considerable progress made, it is still a challenge to design

and predict new phases a priori. Considerable success has been achieved in the

preparation of homologous series of compounds - in this approach the fundamental

concepts that define the basic building units and how they combine are preserved but the

major problem of decreasing stability and accessibility of higher homologues in a series

still remains and requires attention. The study of physical properties of such materials

requires single-phase samples as the electronic behaviour and magnetic properties are

closely related to the structure. Therefore, it is necessary to establish whether the samples

are indeed monophasic and/or to what extent. The sol-gel technique, a versatile and quite

a low-cost technique which makes possible to synthesize varieties of solid materials.

The structure and other properties of the materials by sol-gel method are unique and

differ appreciably from that of the solid materials by other chemical routes.

The sol–gel process is suitable for producing composite materials of high purity

without multiple steps. The synthesis of metal oxides by sol-gel synthesis has proven

extremely versatile since it allows the formation of a large variety of metal oxides at

relatively low temperatures via the processing of metal salt or metal alkoxide precursors.

6

The structure and composition of nano oxides formed by sol-gel method depend on the

preparation condition, the nature of the precursors, the ion source and pH. It offers

advantages such as: (i) tailor-made materials due to good process control, (ii)

homogeneous multicomponent systems due to mixing in liquid medium, (iii) low

temperature for materials processing.

The transition metal nanocomposite oxides which normally are relatively easy to

prepare with low cost by sol-gel method21 as well as the interesting diverse physical

properties have attracted much attention as the new materials in recent years22

1.7 Applications of nanocomposite oxide materials

.

1.7.1 Catalysis

In the area of catalysis, the main challenge is to prepare new materials that would

be utilized for the selective oxidation of alkanes and also for the conversion of methane

to higher hydrocarbons23

This is a very important issue for future research.

. The development of new electrocatalysts for fuel cells is also a

priority. One of the important clues for developing new materials for emerging needs is

to fine-tune the ion mobility and electronic defects in a solid. The oxygen and

hydrocarbon adsorption, activation and redox processes depend on the structure and

composition of the surface of the solid/oxide and our present understanding of the surface

structures has not reached a state where we can predict the performance of the catalysts.

1.7.2 Nanotechnology

Nanotechnology explores different applications due to miniaturization of devices,

since whose properties linked to the particle size of nanomaterials24 as well as nano

composite metal oxides 25.

7

The particle size of materials affects its properties mainly in two aspects. One of

which is the change in structural characteristics, such as the lattice symmetry and cell

parameters whereas bulk oxides are usually stable systems with well-defined

crystallographic structures. The second one is the presence of under-coordinated atoms

(like corners or edges) or O vacancies in an oxide which should produce atomic

arrangements that differs from the properties in the bulk materials and occupied

electronic states located above the valence band of the corresponding bulk materials,

inducing the chemical activity of the system. These properties of nanostructured oxides

lead to the wide industrial applications such as sorbents, sensors, ceramic materials,

photo-devices, catalysts for reducing environmental pollution. In order to prepare these

nanomaterials, novel synthetic procedures have to be developed. By and large, two

significant strategies (i) top-down, (ii) bottom-up fabrication approaches which are

involved liquid-solid or gas-solid transformations. Furthermore, these materials can be

functionalized by surface and structure modification. The good thermal and chemical

stability of these inorganic materials enable them to be widely used.

1.7.3 Soft Magnetic materials

Soft magnetic materials are used in devices such as in floppy disks, magnetic

recording tape, and magnetic stripes on credit cards because the energy loss must be low

provide a small area within hysteresis loop of magnetic moment (M) vs. magnetic (H).

1.8 Theoretical developments

Theoretical investigations are developed significantly by several methodologies

along with the availability of advanced computers during the last two decades.

Computational methods are developed to suggest the target compositions, structure and

study of the competing properties. It is important to utilize the knowledge garnered from

8

the synthesis along with the advancements obtained from computational approaches

gainfully to meet the demands of the developing area of materials chemistry. The need

for more accurate band structure methods necessitates the development of more efficient

algorithms26. The importance of interfacial interactions observed in quantum dots or

nanothermoelectrics with larger length scales require advancements in the Density

Functional theory (DFT) methods27

Theoretical understanding of the electron transfer processes have not been

developed in great detail and this is another area which requires attention. A major

hurdle in the utilization of theoretical methodologies in solid state chemistry research is

the lack of proper literature describing the usefulness of DFT and other techniques in a

language accessible to non-experts. For a more elaborate account, the article by

Kanatzidis and Poeppelmeier

.

28

Methods based on DFT, when adequately tested can be of sufficiently high

accuracy, affording the possibility of treating a wide variety of systems and sizes, up to a

few hundred atoms for symmetric structures. The main disadvantage of DFT calculations

is that they are time consuming. For this reason, it is very important to complement the

time-consuming DFT methods with more approximate models, which are able to

preselect the most promising structures.

is suggested, which provides an excellent survey of the

current research scenario (both in experiment and theory) of this field with an

international perspective.

1.9 Present work

In the present work, the main aim is focused on to exploit some fundamental

aspects of the new series of Bi2+xAxD4-3xO7+δ (A=Al, Ce, Yb & Ga; D=Ni & Pd)

9

nanocomposite oxides which can be important for a better understanding of physical

phenomena observed in this series in terms of their structure-property relations which

have been studied by various solid state experimental techniques and DFT calculations.

1) To synthesis monophasic new series of Bi2+xAxD4-3xO7+δ

2) To investigate the effects of substitution of magnetic (3d or 4d ions) by

diamagnetic (group III-A metal ions) or by magnetic (rare earth ions) on the magnetic,

electronic and optical properties of the series as a whole. Attention is largely confined to

ascertaining the influence of the substitution on the system.

(A=Al, Ce, Yb, Ga;

D=Ni, Pd) nanocomposite oxide materials by sol-gel technique.

3) To determine the crystal structure obtained with varying characterization

techniques.

4) To interpret the electronic properties obtained by DFT calculations.

10

References

[1] B. Raveau, J. Am. Ceram. Soc. 25 (2005) 1965.

[2] P.H.C. Camargo, Kestur, G Satyanarayana, and F Wypych,

[3] S.D. Haa and S. Ramanathan J. Appld. Phys. 110 (2011) 071101.

Mater.Res. 12 (2009) 1.

[4] O.O. Gutfleisch, M.A. Willard , E. Brück , C.H. Chen , S.G. Sankar and J.P. Liu,

Adv. Mater. 23 (2011) 821.

[5] Y. Xuehua, H. Yucai, Z. Li, C. Fengjing, Y. Yingxia, L. Tao and H. Junhui, Curr.

Nanosci. 7 (2011) 576.

[6]

S.C Tjong, Y-W Mai (Woodhead Publishing Ltd., Cambridge, 2010).

S.C Tjong, “Physical Properties and Applications of Polymer Nanocomposites”, (ed.)

[7] B.M. Reddy, G.K. Reddy, A. Khan, and I. Ganesh J. Mater. Sci. 42 (2007) 3557.

[8] N.H. Due, T.D. Hien, D. Givord, J.J.M. Franse and F.R. de Boer, J. Magn. Magn.

Mater. 124 (1993) 305.

[9] S. Xia, and C-K. Duana, J. Lumin. 122–123 (2007) 1.

[10] E. Burzo, L. Chioncel, R. Tetean and O. Isnard J. Phys.: Condens. Matter. 23 (2011)

026001.

[11] J.S. Lee, Y.S. Lee, T.W. Noh, K. Char, J. Park, S.J. Oh, J.H. Park, C.B. Eom, T.

Takeda and R. Kanno, Phys. Rev. B 64 (2001) 245107.

[12] T. Taniyama, E. Ohta and T. Sato, Europhys. Lett, 38 (1997) 195.

[13] C. Xia, H. Ding, C. Shen, T. Yang, C. Hui, and H.-J. Gao, J. Phys. Chem. C

113 (2009) 13466.

[14] M. Ren, C. Zhang, P. Li, Z. Song and X. Liu, J. Magn. Magn. Mater. 324 (2012)

2039.

11

[15] A. Fuerte, M. D. Hernandez-Alonso, A. J. Maira, A. Martinez- Arias, M. Fernandez-

Garcia, J. C. Conesa and J. Soria, Chem. Commun., (2001) 2718.

[16] D. Mocatta, G. Cohen, J. Schattner, O. Millo, E. Rabani and U. Banin, Science, 332

(2011) 77.

[17] Y. C. Cao, Science, 332 (2011) 48.

[18] X. Zhou, J. Shi and C. Li, J. Phys. Chem. C, 115 (2011) 8305.

[19] J. Gopalakrishnan, S. Uma, K. Kasthuri rangan and N.S.P Bhuvanesh, Proc. Indian

Acad. Sci. (Chem. Sci.), 106 (1994) 609.

[20] L. Liu, L. Fang, Y. Huang, Y. Li and D. Shi, J. Appl. Phys. 110 (2011) 094101.

[21] L.L. Hench and J.K. West, Chem. Rev. 90 (1990) 33.

[22] A.R. West, Basic Solid State Chemistry. (2nd edn. John Wiley and Sons,

Chichester, 1999).

[23] J.A. Lercher and F.N. Naraschewski, Nanoscience & Nanotechnology

(Nanostructured Catalysts), 19 (2011) 5.

[24] J.L. Dorman and D. Fiorani, Magnetic Properties of fine particles (North Holland:

Amsterdam, 1992).

[25] Nanorods, Nanotubes and Nanomaterials Research Progress (Nova Science

Publishers, Inc., Hauppauge, N. Y, 2008)

[26] J.R. Chelikowsky, M.L. Tiago, Y. Saad and Y. Zhou Comput. Phys. Commun.

177 (2007) 1.

[27] J.L. Alonso, A. Castro, P. Echenique and A. Rubion, Condensed Matter (2011), 1.

[28] M.G. Kanatzidis and K.R. Poeppelmeier Prog. In Solid State Chem. 36 (2007) 1.

12

Chapter - II

Theoretical background

This chapter describes the experimental procedures and the techniques

employed in this investigation are descried with necessary theoretical background and the

theoretical aspects of magnetism, and density function calculations (DFT calculations).

2.1 X-ray Diffraction Studies

X-ray diffraction (XRD) a solid state technique which used to investigate the

single or multi phases present in the sample and to determine the calcination induced

solid-solid phase transformations if any. X-rays are quite enough to penetrate into the

material and their wavelengths matches with the interatomic distances in solids due to

same order of magnitude of wavelenghths. Thus, a collimated beam of X-rays is

diffracted by the crystalline phases in the sample according to Bragg’s Law1-3

The spacing between two planes (hkl), d, is related to the diffraction angle 2θ by

the Bragg law:

. Powder

diffraction profiles are achieved by measuring the angles 2θ. Position, intensity, shape

and width of the diffraction lines give information about the sample.

nλ = 2d sin θ … (2.1)

Where, λ is the wavelength of the X-rays, d is the distance between two atomic

planes in the crystalline phase, n is the order of the diffraction, and θ the incoming

diffraction angle.

The original phase whether it is single or multi phase can be done by comparing

the set of reflections of the profile with that of pure references phases, or with the

database (powder diffraction file4, (PDF)) provided by International Centre for

13

Diffraction Data (ICDD) (formerly ASTM then JCPDS). It is a collection of single phase

X-ray powder diffraction patterns in the form of tables of characteristic interplanar

spacings and corresponding relative intensities along with crystallographic properties.

2.2 Crystallite Size Determination

The average crystallite size in various samples was estimated with the help of

Scherrer equation using the XRD data of all prominent lines. Diffraction lines should, in

principle, be very narrow; however, when the crystallite size of a polycrystalline material

falls below 1000 Å, broadening of the diffraction lines is observed. Instrumental

limitations and lattice strain can also cause line broadening.

The relation between line broadening and crystallite size for a stress free material,

is given by Scherrer formula:

Kλhkl P

hkl

β = ______________ … (2.2)

hkl

Cos θ

Where K is a constant that depends on the definition of β [FWHM value of a

particular hkl plane, in degrees (a Polar co-ordinate), needs to be converted into radians

(a Cartesian co-ordinate), since, crystallite size measures in length units] used and the

crystal geometry. The constant K varies from 0.98 to 1.39, but because of experimental

uncertainties, the constant is often set equal to 1. When β is defined as the integral

breadth, Phkl is a volume average crystallite size1

2.3 Scanning Electron Microscopy

.

Scanning electron microscopy (SEM) is a versatile technique that can provide, the

wide range of information the SEM is often the preferred preliminary tool for analytic

microscopy. The focused high-energy electrons beam scanned over the materials and

14

interacts with the material, causing a variety of signals; secondary electrons,

backscattered electrons, X-rays, photons, etc. each of which may be used to characterize

a material with respect to specific properties. In order to improve the measurement

precision and improve the electrical conductivity, sometimes samples are coated by Au.

2.4 Energy dispersive X-ray Analysis (EDX)

This analytical technique is often used in conjunction with SEM. An electron

beam typically (10-20 keV) strikes the surface of a conducting sample, causing X-rays to

be emitted, whose energies depend on the material under examination. The X-rays are

generated in a region about 2 ím in depth. EDX is a high-resolution (with approximately

1.5 nm lateral resolution) variant of electron microprobe analysis or X-ray microanalysis

whereby information can be obtained on the chemical composition of individual

nanoparticles.

2.5 Thermal Studies

Thermal studies are a simple analytical technique that deals the thermal reactions

occurred within the materials. The principle of thermal studies includes to measure

weight changes, thermal stability and composition. Thermogravimetric analysis (TGA)

measures weight changes as a function of temperature or time. As samples are heated,

they can lose weight from the process such as evaporation, or from chemical reactions or

sometimes they can gain weight by chemical reactions. Since the magnitude and

temperature range of those reactions are necessary in order to design adequate thermal

ramps and holds during those critical reaction periods. Thermal analysis relies on a high

degree of precision in three measurements which are weight, temperature and

temperature change. The weight loss curves in the measurements look similar, since it

15

requires transformation before results may be interpreted. A derivative from the weight

loss curve can be used to interpret the point at which weight loss occurred.

Again, interpretation is limited without further modifications and deconvolution

of the overlapping peaks may be required. TGA is used to determine characteristics of

materials, to determine degradation temperatures, absorbed moisture content of materials,

the level of inorganic and decomposition points of explosives and solvent residues.

Simultaneous TGA-DTA/DSC measures both heat flow and weight changes (TGA) in a

material as a function of temperature or time in a controlled a transition metal oxides

sphere. Simultaneous measurement of these TGA-DTA/DSC properties improves

productivity as well as simplifies the interpretation of the results. DTA/DSC traces alone

allow differentiation between endothermic and exothermic events which have no

associated weight loss e.g., melting and crystallization and those which involve a weight

loss e.g., degradation.

2.6 UV-vis Diffuse Reflectance Spectroscopy

The absorption bands occurring in the visible and near-UV regions are used to

obtain information on the electronic structure of catalyst and support. Most

heterogeneous catalysts are opaque powdered samples from which light is reflected and

not transmitted. The radiation reflected from a powdered crystalline surface consists of

two components: the specular component that is reflected from the surface without

transmission (mirror reflection) and the diffuse component that is absorbed into the

material and reappears at the surface after multiple scattering. Commercial

spectrophotometers are designed to minimize the specular component and the term

reflectance is used for diffusely reflected radiation5-8. The ratio of the light scattered

16

from a 2-3 mm thick layer of sample and an ideal non-absorbing reference sample is

measured as a function of wavelength λ. The relation between the diffuse reflectance of

the sample (R0), absorption (K), and scattering (S) coefficients are related by the

Schuster-Kubelka-Munk (SKM) remission function6, 7

F(R

as follows

0) = (1- R0)2/2 R0

The samples were pelletized with KBr matrix. The corresponding diluent (KBr)

was used as baseline standard.

= K/S … (2.3)

2.7 Theory of magnetism

When applying a magnetic field to a sample, the sample’s magnetism can be

investigated. A magnetic material can exhibit a diamagnetic or paramagnetic response.

The magnetic susceptibility is proportional to the magnetisation of the sample and

inversely proportional to the strength of the field (2.4). When the magnetisation opposes

the magnetic field, the magnetic susceptibility is less than zero and is called

diamagnetism. Then the magnetisation is added to the field, the magnetic susceptibility is

positive and is called paramagnetism.

Diamagnetic material will only have a magnetic response when an external field

is applied. However, paramagnetic material will have a magnetic response even in the

absence of an applied field. The material tends to have a permanent dipole moment,

which is commonly seen in metals due to the unpaired electrons in the atomic orbital. It is

due to this unpaired electron response that helium, which has only paired electrons, is

used as the gas instead of oxygen, which has two unpaired electrons, when studying the

magnetism of the material. When a material is purely paramagnetic, the individual

dipoles from the atoms will not interact with each other and are entirely random in

17

orientation. When the dipole moments do interact, ferromagnetism and

antiferromagnetism is the result.

𝜒𝜒 = 𝑀𝑀𝐻𝐻

… (2.4)

𝜒𝜒 = 𝐶𝐶

(𝑇𝑇 − 𝜃𝜃)=

𝜇𝜇2

8(𝑇𝑇 − 𝜃𝜃) … (2.5)

Where,

𝐶𝐶 = 𝜇𝜇2

8 … (2.6)

In ferromagnetism, the dipoles align with each other, with the spins being in the

same direction, in the absence of an applied field. Since electrons that are aligned are

spinning in the same direction, there is a small repulsion that will lower the electrostatic

energy and this leads to nearby electrons also aligning with these electrons. This

alignment is only short range though and over a longer range a neighbouring region may

be antialigned. When neighbouring dipoles antialign, this is different to ferromagnetism

and is called antiferromagnetism.

Antiferromagnetism is less common than ferromagnetism, and occurs at a lower

temperature. As the temperature decreases, the antiferromagnetic response can typically

become diamagnetic and also exhibit some ferromagnetic behaviour. Both ferromagnetic

material and antiferromagnetic material will spontaneously align or antialign below a certain

temperature. Above this temperature, both types of magnetism tend to become paramagnetic,

as the mobility of spin increases and therefore spontaneous alignment cannot occur. For

ferromagnetic material, this is the Curie temperature and for antiferromagnetic material, it is

the Néel temperature, which is analogous to each other. A magnetometer can be used below

these temperatures to apply a field to study the materials behaviour. Magnetic coils are

18

assigned a magnetic field and the sample is placed between them. By moving the sample in

and out of these fields, the magnetic susceptibility can be calculated. The magnetic

susceptibility (χ) is equal to the magnetism of the sample (M) divided by the applied field

(H), (2.4). The magnetic susceptibility can also be seen by using the Curie-Weiss law (2.5).

Using this law, the magnetic susceptibility is a constant (C) divided by the temperature

(T) in Kelvin minus the sign and strength of the coupling (θ), where the constant C is the

square of the magnetic moment (μ) divided by 8 (2.6). By altering the temperature, the

susceptibility will increase.

2.8 DFT Calculations

The general idea of DFT is that all properties of the ground state of a system can

be considered to be functional of the ground state density of particles 9-11

The Hamiltonian of N electrons in an external potential 𝑉𝑉𝑒𝑒𝑒𝑒𝑒𝑒 (𝑟𝑟) can be written as:

Ĥ = −ℏ2

2𝑚𝑚�∇𝑖𝑖2𝑁𝑁

𝑖𝑖

+ �𝑉𝑉𝑒𝑒𝑒𝑒𝑒𝑒 (𝑁𝑁

𝑖𝑖

𝑟𝑟𝑖𝑖��⃗ ) +12�

𝑒𝑒2

�𝑟𝑟𝑖𝑖��⃗ − �𝑟𝑟𝑗𝑗��⃗ ��𝑖𝑖≠𝑗𝑗

… (2.7)

where, the first term is a sum operator of kinetic energy of electrons, the second

term is a sum of 𝑉𝑉𝑒𝑒𝑒𝑒𝑒𝑒 (𝑟𝑟), where in the case of a crystal

𝑉𝑉𝑒𝑒𝑒𝑒𝑒𝑒 (𝑟𝑟) = −�e2Zj

�𝑟𝑟𝑖𝑖��⃗ − �𝑅𝑅𝑗𝑗��⃗ �� … (2.8)

M

i≠j

.

is a potential of Coulomb interaction of i-electron with nuclei, and the third term is the

Coulomb electron-electron interaction. DFT is based upon two theorems formulated by

Hohenberg and Kohn9

Theorem 1. The expectation value of any observable is a unique functional of the

exact ground state electron density 𝑛𝑛𝑜𝑜(𝑟𝑟). All ground state properties can thus be

.

19

extracted from the exact electron density. The problem is now only how to find this

density. The second theorem is helpful in this matter.

Theorem 2. For a given external potential, the true density 𝑛𝑛𝑜𝑜(𝑟𝑟) minimizes the

total energy functional. This reduces the very complex problem of finding all physical

properties of a system to finding the minimum of the energy with respect to the electron

density (which still is not trivial since 𝑛𝑛𝑜𝑜(𝑟𝑟) is function in three-dimensional space).

Therefore, the total energy of the systems is a functional of electron density and can be

written as:

𝐸𝐸𝑒𝑒𝑜𝑜𝑒𝑒 𝑜𝑜 [𝑛𝑛(𝑟𝑟)] = 𝑇𝑇[𝑛𝑛(𝑟𝑟)] + 𝐸𝐸𝑒𝑒𝑒𝑒𝑒𝑒 [𝑛𝑛(𝑟𝑟)] + 𝐸𝐸𝑒𝑒𝑒𝑒 [𝑛𝑛(𝑟𝑟)] + 𝐸𝐸𝑒𝑒𝑥𝑥 [𝑛𝑛(𝑟𝑟)] … (2.9)

where 𝑇𝑇[𝑛𝑛(𝑟𝑟)] is the total kinetic energy of a system of non-interacting electrons with

density

𝑛𝑛(𝑟𝑟),𝐸𝐸𝑒𝑒𝑒𝑒𝑒𝑒 [𝑛𝑛(𝑟𝑟)] = �𝑉𝑉𝑒𝑒𝑒𝑒𝑒𝑒 (𝑟𝑟) 𝑛𝑛𝑉𝑉

(𝑟𝑟)𝑑𝑑3𝑟𝑟 … (2.10)

describes the electron-nuclei interaction, the so-called Hartree term,

𝐸𝐸𝑒𝑒𝑒𝑒 [𝑛𝑛(𝑟𝑟)] = 𝑒𝑒2

2�

𝑛𝑛(𝑟𝑟)𝑛𝑛(𝑟𝑟′)|𝑟𝑟 − 𝑟𝑟′|

𝑉𝑉

𝑑𝑑3𝑟𝑟𝑑𝑑3𝑟𝑟′ , … (2.11)

is the classical energy of the Coulomb electron-electron interaction, and

𝐸𝐸𝑒𝑒𝑥𝑥 [𝑛𝑛(𝑟𝑟)] = ∫ 𝑉𝑉𝑒𝑒𝑥𝑥 [(𝑟𝑟),𝑛𝑛(𝑟𝑟)]𝑉𝑉 𝑑𝑑3𝑟𝑟 … (2.12)

is the exchange-correlation energy functional which compensates for the electron-

electron interaction being described only by the Hartree term 𝐸𝐸𝑒𝑒𝑒𝑒 [𝑛𝑛(𝑟𝑟)].

These theorems show is that a unique mapping between the ground state density

and the ground state energy in principle exists. In fact, in (2) only Eee is known, while the

20

explicit forms of the other contributions are unknown. Kohn and Sham invented an

ingenious indirect approach to the kinetic-energy functional, which rests on two

assumption10

The density of the auxiliary system is given by sums of squares of the wave function of

: (1) the exact ground state density can be presented by ground state density

of an auxiliary system of non-interacting electrons, and (2) the Hamiltonian is chosen to

have the usual kinetic operator and an effective local potential acting on an electron of

spin s at point 𝑟𝑟.

electrons:

- Without spin: 𝒏𝒏(𝒓𝒓�⃗ ) = ��𝝍𝝍𝝁𝝁(𝒓𝒓�⃗ )� 𝟐𝟐

𝝁𝝁

, … (𝟐𝟐.𝟏𝟏𝟏𝟏)

- With spin:

𝑛𝑛(𝑟𝑟) = �𝑛𝑛(𝑠𝑠

𝑟𝑟, 𝑠𝑠) = � ��𝜓𝜓𝜇𝜇𝑠𝑠(𝑟𝑟)�2

𝜇𝜇

𝑠𝑠

, … (2.14)

- For 𝐸𝐸𝑒𝑒𝑜𝑜𝑒𝑒𝑜𝑜 [𝜓𝜓𝜇𝜇 (𝑟𝑟)] to be a minimum, it is necessary that 𝛿𝛿𝐸𝐸𝑒𝑒𝑜𝑜𝑒𝑒𝑒𝑒

𝜓𝜓𝜇𝜇∗ (𝑟𝑟) = 0 with leads to

the Kohn-Sham equations:

�− ℏ2

2𝑚𝑚 𝛻𝛻2 + 𝑉𝑉𝑒𝑒𝑒𝑒𝑒𝑒 (𝑟𝑟) + e2 ∑ ∫ |𝜓𝜓𝑖𝑖(𝑟𝑟)|2

|𝑟𝑟−𝑟𝑟′| d3r′ + Vxc [𝑟𝑟,𝑛𝑛(𝑟𝑟)]i � 𝜓𝜓𝜇𝜇 (𝑟𝑟) = 𝜖𝜖𝜇𝜇𝜓𝜓𝜇𝜇 (𝑟𝑟) … (2.15)

Here m e are Lagrange multipliers for the constraints∫𝜓𝜓𝜇𝜇∗(𝑟𝑟)ψv (𝑟𝑟)d3r = δμv ,

and are interpreted as one-electron energies.

𝑉𝑉𝑒𝑒𝑥𝑥 [𝑟𝑟; n] ≡ 𝜕𝜕𝐸𝐸𝑒𝑒𝑥𝑥 [𝑟𝑟; n]𝜕𝜕𝑛𝑛(𝑟𝑟) … (2.16)

21

is the exchange-correlation potential. These equations are nonlinear, self-consistent, and

must be solved iteratively. Total energy of the ground state depends on m e and the wave

function (r ) r m y of the one-electron states:

𝐸𝐸𝑒𝑒𝑜𝑜𝑒𝑒 = �𝜀𝜀𝜇𝜇 −𝑒𝑒2

2𝜇𝜇

�𝑛𝑛(𝑟𝑟)𝑛𝑛(𝑟𝑟′)

|𝑟𝑟 − 𝑟𝑟′|𝑑𝑑3𝑟𝑟𝑑𝑑3𝑟𝑟′ −

14�𝑉𝑉𝑒𝑒𝑥𝑥 (𝑟𝑟;𝑛𝑛)𝑛𝑛(𝑟𝑟)𝑑𝑑3𝑟𝑟, … (2.17)

The early electronic structure calculations based on local spin density

approximation (LSDA) methods generally fails when employed for strongly correlated

systems such as transition metal oxides12. The realistic description of the electronic

structure of magnetic systems containing strongly correlated localized electrons can be

treated by a modified approach of using a combination of LSDA and Hubbard ‘U’

method (LSDA + U)13 and the self-interaction corrected pseudo-potential method

(pseudo-SIC)14. The two methods, though different, yield band gaps and band structures,

magnetic moments and crystal structures that are in reasonable agreement with the

experimental findings. The quantum density functional theoretical (DFT) methods,

generally, are limited to 1000 atoms and ~10−12

Recently, computer intensive methods, such as the LDA + DMFT and the GW,

have been developed for more rigorous physical foundation. The developments of the

first-principles description of strongly correlated materials along with the emergence of

‘theory of polarizations’ have helped in the conceptual understanding of spontaneous

polarizations in bulk crystals (complicated by the periodicity of the crystal lattice) within

the density functional framework

s time scales and the tight binding

methods are limited to <105 atoms. The use of classical interatomic potential help in

simulating about million times larger size both in number (space) and time as such large

systems are required to study the problems of interest in solid state chemistry.

15. These developments have helped in the calculations

22

of ferroelectric polarizations, dielectric constants, piezoelectric coefficients and the

electric field responses in a solid, routinely. The development of first principles method

for calculating metal-insulator heterostructures is another important development. The

advancements in the ab initio methods will be very beneficial for understanding the

electronic and structural study of solids. The quantum density functional theoretical

(DFT) methods, generally, are limited to 1000 atoms and ~10−12 s time scales and the

tight binding methods are limited to <105 atoms. The use of classical interatomic

potential help in simulating about million times larger size both in number (space) and

time as such large systems are required to study the problems of interest in solid state

chemistry. The growing use of molecular modeling (MM) in combination with the

quantum mechanics (QM) could help in arriving at better results. The QM methods can

be applied for studying the chemical reactions, the change of coordination numbers

whereas MM methods can be applied for classical interactions. Thus, the QM-MM

approaches would require careful consideration in the incorporation of QM methodology

within the MM surroundings. By combining molecular dynamics (MD) with Monte-

Carlo techniques employing the bond order dependent potentials, one is in a position to

investigate the many issues associated with materials chemistry16. The theoretical

advancements over the years have been employed in the areas of synthesis, in

understanding phase stabilities in multicomponent systems, kinetic studies, magnetic

behavior, spin polarizations in complex solids, thermoelectrics and in multi-functional

solids. The tremendous advancement in utilizing the density functional methodology

along with the computational power, the electronic structure theory has now become an

integral part of research in solid state chemistry. The electronic structure theory helps not

23

only in the interpretation and rationalization of experimental results, but also in our

understanding of the crystal structure-property relationships. It is important that the

theoretical methods need to be developed from being an analytical tool into a more

predictive instrument to make computationally designed materials. The DFT methods

used for determining band gaps, elastic and other related properties may be improved to

study a combination of properties via high throughput methods. The inability of the DFT

to handle the van der Waals interactions and hydrogen bonds in solids is another area

where developments are required. The non-local functionals, which are computationally

intense, need improvements to describe many of the properties of layered solids such as

graphite, MoS2

, etc.

24

References

[1] H.P. Klug and L.E. Alexander, X-Ray Diffraction Procedures for Polycrystalline and

Amorphous Material, (2nd ed., Wiley, New York, 1974).

[2] P. Gallezot “Catalysis: Science and Technology” (Eds.) J.R. Anderson and

M.Boudart, (Springer, Berlin, 1984).

[3] G. Bergeret, P. Gallezot “Catalyst Characterization: Physical Techniques for

Solid Materials) (Eds.) B. Imelik, J.C. Vedrine (Plenum Press, New York, 1994).

[4] International Centre for Diffraction Data, 12 Campus Boulevard, Newton Square,

PA 19073-3273, USA.

[5] B.M. Weckhuysen and R.A. Schoonheydt, Catal. Today 49 (1999) 441.

[6] G. Kortum, “Reflectance Spectroscopy: Principles, Methods and Applications”,

(Springler Verlag, Berlin, 1969).

[7] W.N. Delgass. G.L. Haller, R. Kellerman, J.H. Lunsford, “Spectroscopy in

Heterogeneous Catalysis”, (Academic Press, New York 1979).

[8] X. Gao, I.E. Wachs, J. Phys. Chem. B 104 (2000) 1261.

[9] P. Hohenberg and W. Kohn, Phys. Rev. 136, B864 (1964).

[10] W. Kohn and L. J. Sham, Phys. Rev. A, 140 (1965) 1133.

[11] W. Kohn, Rev. Mod. Phys. 71, 1253 (1999).

[12] N.A Hill, J. Phys. Chem. B 104 (2000) 6694.

[13] V.I. Anisimov, F. Aryasetiawan and A. I. Lichtenstein J. Phys. Cond. Mater. 9

(1997) 767.

[14] A. Felippetti and N.A. Spaldin Phys. Rev. B 67 (2003) 125109.

[15] R. Resta, Rev. Mod. Phys. 66 (1994) 899.

[16] Q. Zhang, Y. Qi, Jr. L.G. Hector, T. Cagin and W.A. Goddard III, Phys. Rev. B 72

(2005) 045406.

25

CHAPTER-III

Experimental

In this chapter, the various experimental methods used for the preparation and

characterization of the polycrystalline samples are described. In addition to the

preparation procedures, the characterization techniques include X-ray diffraction (XRD),

Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) analysis,

Thermal analysis (DSC/DTA/TGA), Infra-red spectroscopy (IR), EPR studies at various

temperatures, Magnetic susceptibility, optical absorption studies, Chemical analysis and

density measurements.

3.1 Sample Preparation

Reagent grade chemicals were used to prepare the samples by sol-gel method.

Calculated amounts of bismuth nitrate, Bi(NO3)3, nickel nitrate, Ni(NO3)2, palladium

nitrate, Pd(NO3)2, aluminium nitrate Al(NO3)3, cerium nitrate Ce(NO3)3, ytterbium

acetate Yb(OCOCH3)3 and gallium nitrate Ga(NO3)3

were dissolved were dissolved in

acidified (HNO3) water to prepare a in water to prepare 0.1 molar solution of pH ~ 2

each. The solutions were mixed together and stirred for 1 h at ~ 50 °C, then ~30 ml 1.5 M

solution of citric acid was added and stirred continuously at ~ 50 °C for ~ 85-90 h which

then forms a waxy gel. The resulting gel was then decomposed at ~ 120 °C, which was

then sintered in air successively at 450 ºC for 2 h for complete combustion of organic

materials, and at 800 °C (A), 650 ° C (B & C), 750 ° C (D)for 4 h and quenched in air to

obtain fine green(A & B) and dark brown(C&D) powder.

26

Table 3.1 Compositions of samples in the series of Bi2+xAxD4-3xO7+ δ

Ga; D=Ni & Pd) nanocomposite oxide (A=Al, Ce, Yb &

___________________________________________________________ System Sample code x composition ___________________________________________________________

A1 0.00 Bi2.0Ni4.0O

A A2 0.25 Bi7

2.25Al0.25Ni3.25O

A3 0.50 Bi7

2.5Al0.5Ni2.5O

A4 0.75 Bi7

2.75Al0.75Ni1.75O7

B1 0.25 Bi2.25Ce0.25Ni3.25O

B B2 0.50 Bi7

2.5Ce0.5Ni2.5O

B3 0.75 Bi7

2.75Ce0.75Ni1.75O

B4 1.00 Bi7

3.0Ce1.0Ni1.0O

7

C1 0.00 Bi2.0Pd4.0O

C C2 0.25 Bi7+ δ

2.25Yb0.25Pd3.25O

C3 0.50 Bi7

2.5Yb0.5Pd2.5O

C4 0.75 Bi7

2.75Yb0.75Pd1.75O7

D1 0.15 Bi2.15Ga0.15Pd3.55O

D D2 0.30 Bi7

2.3Ga0.3Pd3.1O

D3 0.45 Bi7

2.5Ga0.5Pd2.65O

D4 0.60 Bi7

2.6Ga0.6Pd2.2O

_______________________________________________________________ 7

27

Fig. 3.1. Flowchart of the synthetic strategy of the samples in the series of Bi2+xAxD4-3x

O7+ δ

(A=Al, Ce, Yb & Ga; D=Ni & Pd) nanocomposite oxide

28

3.2 X-ray Diffraction Studies

The X-ray diffraction patterns of the powder samples were recorded on a X’pert

(PAN Analytical) diffractometer. The diffraction patterns of the samples were obtained

using monochromatic Cu Kα X-radiation of wavelength, λ ~ 1.54056 Å from ceramic

tube. The X-ray tube was operated at 25 mA/ 40 kV with 1000 Watts X-ray power. The

diffraction patterns were obtained in the range 5-80 0 in 2θ for all samples. The count rate

was maintained at 1000 counts per second using a proportional counter, with time

constant in second in slow mode. From the XRD pattern of the polycrystalline solids, the

average crystallite sizes, Phkl, were determined by using the following Scherrer equation1

3.3 Scanning Electron Microscopy and Energy Dispersive X-rays

.

𝑃𝑃ℎ𝑘𝑘𝑘𝑘 =𝐾𝐾𝐾𝐾

𝛽𝛽12𝐶𝐶𝑜𝑜𝑠𝑠𝜃𝜃ℎ𝑘𝑘𝑘𝑘

… (3.1)

Where K is a constant = 1.0, λ is the wavelength of the radiation (λ=1.54818Å)

and 𝛽𝛽12 is the half peak width.

The room temperature surface morphology of the samples were examined by

scanning electron microscopy (SEM)2. In all cases, more than one image is taken from

neighbouring areas to verify the true representation of the morphology of the samples.

The SEM images are taken from “HITACHI-S3400N” and Energy Dispersive X-rays

(EDX) done by ‘Super dryer-II’

3.4 Thermal Studies

model analyser.

The Differential scanning calorimetry (DSC) 3, 4 is a thermoanalytical technique in

which the variation of heat required to enhance the temperature of a sample and reference

are measured as a function of temperature. The thermal studies traces of nanocomposite

29

oxides of system-A were recorded on a DTA/TGA/DSC analysis was done using a

Perkin-Elmer Diamond TG/DTA instrument at SAIF, IIT, Madras (sensitivity of 0.2 μg)

in the range 25- 800 °C at heating rate of 25°C/min in air. Nanocomposite oxides of

System-B, -C, -D were recorded on SDT Q600 V20.9 Build 20 in the temperature range

20-1000 o

3.5 FT-IR Spectroscopy

C at CIF Pondicherry University.

The IR spectrum of the glasses was recorded at room temperature in the range 400-

4000 cm-1 by a SHIMADZU FTIR-8700 spectrometer using “KBr pellet” technique 5,6

3.6 EPR Spectroscopy

.

In modern instruments, the absorption spectrum is measured indirectly using a Michelson

interferometer to record the interference fringes produced by combining the radiation

transmitted by sample to radiation transmitted along a variable path within the

interferometer. In the FTIR spectrometer high sensitivity pyroelectric detector and

ceramic beam source were used. All the parameters except the weight of the specimens

were maintained constant for recording the spectra.

X-band EPR spectral measurement at 300 K and 77 K were done with a JEOL JES-

TE 100 ESR spectrometer system with 100 kHz field modulation and a phase sensitive

detector to obtain first derivative signal.Quartz sample tubes were used for recording the

EPR spectra of powder specimens. In order to measure the line width variation as a

function of composition in the samples the line shapes were recorded with sufficiently

low microwave power to avoid saturation. The magnetic field calibration have been made

with respect to the resonance line of DPPH (gDPPH = 2.00354)7 which is used as a field

marker.

30

At liquid helium temperature (5 K) EPR experiments were performed at X-band on

a Varian E-122 spectrometer. Samples were run at low temperatures using an Air

Products Helitran cryostat. The magnetic field was calibrated using with a Varian NMR

Gauss meter and the microwave frequency was determined using an EIP frequency

meter. Since a Gauss meter and frequency meter were used to record the line shapes in

these cases, a g-standard is not necessary. The g-factors can be computed directly from

the spectra using the recorded value of the frequency and the field position.

3.7 Magnetic Susceptibility Measurements

Magnetic measurements were made using EG and G Model 155 Vibrating Sample

Magnetometer (VSM). Magnetic moments of the samples in emu/g were obtained at

room temperature by applying magnetic fields from -12 to +12 kG. The static

susceptibility, χ was obtained using the following equation.

M = χ H … (3.2)

where M is the magnetic moment (emu/g) and H is the applied magnetic field. The Weiss

constant, Δ, is obtained from the following equation8

where N is the number of the spins in the sample, g is the Lande g-factor, β

.

𝜒𝜒 =𝑁𝑁𝑔𝑔2𝛽𝛽𝑒𝑒2𝑆𝑆(𝑆𝑆 + 1)

3𝑘𝑘(𝑇𝑇 + ∆) … (3.3)

e

The exchange integral, j, between two paramagnetic ions are obtained by the

approximation

the

Bohr magneton, S is the electronic spin, k is the Boltzman constant and T is the absolute

temperature.

9.

31

∆=�2

3� 𝑧𝑧𝑆𝑆(𝑆𝑆 + 1)𝑗𝑗𝐾𝐾

… (3.4)

Where z is the number of paramagnetic nearest neighbor ions exchange coupled

to each paramagnetic ion.

Magnetic susceptibility data of the samples A1 and A4 in the range 5-350 K were

registered with a SQUID magnetometer (Quantum Design) in an applied magnetic field

of 8000 Oe.

3.8 Optical Absorption Studies

The room temperature optical absorption spectra in the 200-800 nm region was

measured by using a Shimadzu, model UV 3100 spectrophotometer. An electronic

transition between orbitals that are centred on different atoms is called a charge transfer

transition10, 11

3.9 Determination of Density

and the absorption band is usually very strong. When an electron moves

from orbitals that are predominantly ligand in character to orbitals that are

predominantly metal in character then the transition is classified as a ligand-to-metal

charge transfer (LMCT) transition. When the charge migration occurs from orbitals that

are predominantly metal in character to orbitals that are predominantly ligand in character

then the transition is classified as a metal-to-ligand charge transfer (MLCT) transition.

The densities of the various specimens were determined by using a density bottle

at room temperature. A known weight, W1 of the powdered sample was taken in the

density bottle which was then about half filled with carbon tetrachloride. This was kept as

such for an over-night. Then the bottle was completely filled with carbon tetrachloride

and then weighed. The process was repeated at least three times and average weight W2

32

was taken. In the second experiment the density bottle was filled with carbon

tetrachloride only and the average weight W3

Where d

was determined using a single pan balance.

The density, d, was calculated according to the relationship

𝑑𝑑 =𝑊𝑊1

𝑊𝑊3 + 𝑊𝑊1 −𝑊𝑊2 𝑋𝑋 𝑑𝑑1 … (3.5)

1

3.10 Band structure and density of states calculations

is the density of carbon tetrachloride (1.594 g/cc at room temperature).

The calculations of electronic energy band structures and density of states were

done using a plane-wave DFT theory with local gradient-corrected exchange-correlation

functional12 and performed with a commercial version of the CASTEP (Cambridge Serial

Total Energy Package) programme package13, 14 using Material Studio (MS) software

which uses a plane-wave basis set for the valence electrons and norm-conserving

pseudopotential15

for the core states.

33

References :

1. S. Bera and R.N.P. Choudhary, Bull. Mater. Sci. 19 (1996) 1083.

2. D.A. Skoog, F. J. Holler and T. A. Nieman, “Principles of Instrumental Analysis” (5th

3. Ref. 2 p. 805-808.

ed.), (Eastern Press, Bangalore, 2005) p.550-553.

4. H.H. Willard, L.L. Merritt, Jr., J. A. Dean and F. A. Settle, Jr, “Instrumental Methods

of Analysis” (7th

5. W. Brugel, “Introduction to IR Spectroscopy” (John Wiley and Sons., Inc., New

York, 1962) p. 228.

ed.), (Cbs Publishers and Distributers, New Delhi, 1986).

6. Ref. 2, p. 407.

7. M.B. Hocking and S.M. Mattar, J. Magn. Reson. 47 (1982) 187.

8. C.P. Poole, Jr., “Electron Spin Resonance” (Interscience, New York, 1967).

9. J.W. Dodd and K.H. Tonge, “Analytical Chemistry by Open Learning-Thermal

Method”

10. D.F. Shriver, P.W. Atkins and C.H.

(John Wiley and Sons, Inc., New York, 1987).

Langford “Inorganic Chemstry”

11. B.N. Figgis,

(Oxford

University Press, 1990).

“Introduction to Ligand Fields”

12. J. P. Perdew, K Burke and M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865.

(JohnWiley and Sons, Inc., New York,

1966).

13. M. Segall, P. Lindan, M. Probert, C. Pickard, P Hasnip, S. Clark and M. Payne,

Materials Studio CASTEP, version 2.2, 2002.

14. M.D. Segall, P.L.D. Lindan, M.J. Probert, C.J. Pickard, P.J. Hasnip, S.J. Clark, and

M.C. Payne, J. Phys.: Cond. Matt. 14 (2002) 2717.

15. D.R. Hamann, M. Schluter, and C. Chiang Phys. Rev. Lett. 43 (1979) 1494.

34

CHAPTER – IV

Structure-Property Relations in New Bi2+xAlxNi4-3xO7+ δ

4.1 Introduction

(0.0 ≤ x ≤ 0.75) Series

The study of physical properties of oxides requires single-phase samples because

electrical transport and magnetic properties are closely related to the structure. Therefore,

it is necessary to establish whether the samples are really monophasic. Of special interest

to us are the novel hybrid metal-ceramic materials prepared by sol-gel technique1. This

technique has enabled the preparation of variety of interesting solid materials including

ceramic powders and glasses1,2. Located at the center of the transition element series,

Ni2+ is good trap of photogenerated hole converting it to Ni3+, which is good oxidant of

organic molecule through formation of carbocataion3. The holes associated with a

quadrupole complex contribute to conductivity by hopping and/or by band like

conduction depending on various parameters like temperature, frequency of the applied

signal, defect concentration, and dopant concentration, if any4

New series of oxide material Bi

. The nature of the highest

occupied bands, responsible for the electronic properties of a material, is strongly

dependent on both its composition and its crystal structure.

2+xAlxNi4-3xO7+δ (0.0 ≤ x ≤ 0.75) using sol-gel

method via nitrate-citrate route and found the crystal structure, cubic with the space

group Fm3m by powder XRD pattern and confirmed the structure by rietveld refinement

is same for all the four samples (x = 0.0, x = 0.25, x = 0.5 and x = 0.75). The substitution

effects of the trivalent cations M3+ (Bi3+ and Al3+) on the electronic structure are studied.

The magnetic studies show that the ferro magnetic behaviour at room temperature. The

35

energy band structures and density of states (DOS) are calculated by the plane-wave-

density function theory (DFT).

4.2 Results and Discussion

4.2.1 Powder X-ray diffraction Studies

The powder XRD patterns of samples A1-A4 of Bi2+xAlxNi4-3xO7+δ (0.0 ≤ x ≤

0.75) composite oxide system are shown in Figure 4.1. Using the Fullprof software5 the

unit cell parameters as well as the indexing of the lattice planes in the samples A1-A4 are

done. The unit cell in the samples is found to be cubic with space group Pm3n. The cell

parameters are shown in Table 4.1 along with the observed densities of the samples.

Comparison of the observed density with the calculated ones show that Z=4 for the

samples A1-A4. The cell dimension decreases in A1-A4 with increasing x. This is

attributed to the incorporation of higher valent Bi3+ and Al3+ ions in the lattice. The

average crystallite sizes in the samples are determined by Scherrer’s relation6

It is assumed that the appreciable peak broadening in all the diffraction profile

arises from the crystallite sizes. The broadening of diffraction peaks is due to crystallite

size varies with angle as sec θ and that due to elastic strain as tan θ. The crystallite size

of the prepared compositions is calculated using the Scherrer formula.

, and the

values are also shown in Table 4.1. The crystallite sizes are in the plausible range of 42-

61 nm in the samples.

36

Fig. 4.1 Powder XRD indexed pattern of the system Bi2+xAlxNi4-3xO7+δ

(A1-A4: x=0,

0.25, 0.50, 0.75)

37

Table 4.1 Unit cell parameters, calculated and observed densities and average crystallite sizes in A1-A4 of Bi2+xAlxNi4-3x

Unit cell data/Sample A1 A2 A3 A4

O7 (0.0 ≤ x ≤ 0.75) composite oxides ________________________________________________________________________

________________________________________________________________________ Cell Formula Bi8Ni16O28 Bi9Al1Ni13O28 Bi10Al2 Ni10 O28 Bi11Al3Ni7OLattice parameter* a (Å) 10.2767 10.1197 10.1183 10.1134

28

Cell volume (Å3

Space group Pm3n Pm3n Pm3n Pm3n ) 1085.32 1036.35 1035.90 1034.40

Z 4 4 4 4 Calculated density (g/cc) 4.693 4.944 5.013 5.065 Experimental density (g/cc) 4.398 4.946 5.169 4.825 Average crystallite size (nm) 42 50 45 61 _______________________________________________________________________

* before refinement

4.2.2 Microstructure and EDX

Figs. 4.2.1 (a) - 4.2.4 (a) show the SEM micrographs and Figs. 4. 2. 1 (b) – 4.2.4

(b) show the EDX profiles of A1-A4 respectively. The high magnification SEM

micrograph of A4 is shown in Fig. 4.2.4 (a), which shows particles with quadrupods-like

shape with random orientations. SEM micrographs show that as the concentration of Al

increases the shape of the particles changes from globular shape to quadrupods-like shape

in A1 to A4. Investigation of various regions of the sample also shows similar results

with respect to the particle sizes and shapes. No preferred orientation is observed. This,

very likely, could be ascribed to the quenching as a step in the sample preparation. The

compositions of the samples were characterized by EDX profiles which show that all

samples have the stoichiometry close to nominal. The EDX analysis indicated

homogeneous distribution of all elements, within the grains of the samples. The mapping

of the samples also shows the high dispersion and homogeneity in distribution of

elements in the samples.

38

4.2.3 Thermal Studies

DTA and TG traces show no endothermic peak with weight loss upto 600 ºC

indicating no absorption of moisture or coordinated water in the samples. Rather the

weight increase in TG curve in the range 20-400 °C (~1.2 %) may be resulted from the

oxidation of the nickel present in the system. DTA curves show an endothermic peak

with weight loss at 630 ºC. No other anomalies were observed. This fact indicated that

no any phase transitions upto 800ºC.

4.2.4 Crystal Structure Refinement and Electron Density Fourier Mapping

The unit cell structure (Figure 4.4.1(a-d)) of the samples A1-A4 is developed on

space group Pm3n (sp. gr. No. 223) with Z=4. The Bi3+ ions are put in 8(e) sites in A1

with additional Bi3+ ions (blue colored in Fig. 4.4.1) in A2-A4 in 6(d) sites; Ni2+ (or Al3+)

ions (violet in color (Fig. 4.4.1) in 12(f) and 6(c) sites; O2- ions (red colored in Fig. 4.4.1)

in 16(i) and 12(f) sites. The unit lattice dimensions and the agreement factors after

Rietveld structure refinement7

2-dimensional electron density contour on (010) and (111) plane are shown in

Figures 4.4.2 (a-h). In the asymmetric unit, there is one crystallographically unique Bi

atom, two different O atoms denoted as O1 and O2 and two different atoms of Ni denoted

as Ni1 and Ni2 in A1 and with this atoms one more Bi atom is present in A2-A4, the Bi

atoms denoted as Bi1 and Bi2 Fig 4.4.3 (a-d). In conclusion the progressive substitution

are shown in Table 4.4.1. The Tables 4.4.2-4.4.3 also

show the refined atomic position coordinates, bond lengths and bond angles in the

asymmetric units of A1-A4, respectively.

39

of Ni by Al and Bi remains the cubic Fm3m structure. Bond distances and bond angles

obtained from the refinement for all compositions are listed in Table 4.4.2.

40

Fig. 4.2.1 (a) SEM micrograph at 2 µm magnification and (b) EDX profile of Bi2Ni4O7

(A1) with compositions of the elements in randomly selected grain.

41

Fig. 4.2.2 (a) SEM micrograph at 2 and 5 µm magnifications and (b) EDX profile of

Bi2.25Al0.25Ni3.25O7

selected grain.


42


Bi2.5Al0.5Ni2.5O7

selected grain.


43

Fig. 4.2.4 (a) SEM micrograph at 2 and 1 µm magnification and (b) EDX profile of

Bi2.75Al0.75Ni1.75O7

selected grain.

A4 with compositions of the elements in randomly

44

At first sight, it appears that for the compositions A2-A4, no significant changes

are observed in the bond lengths suggesting that the total bonding interaction is

practically constant in the composition range studied. No significant change in bond

angles is also observed in the samples. This is related to the microstructure of the samples

which show uniform behaviour. The Fourier mapping of the 3-dimensional electron

density from xz-plane (010), and the 2-dimensional electron density contour on (010)

plane are shown in Figures 4.4.4 (a-d), 4.4.5 (a-d) respectively. The circular nature of the

contours around the Ni due to localized valence d orbitals and the O due to valence 2s

and 2p orbitals in the lattice sites with isolated overlapping contours also shows the

partial ionic character of Ni-O bonds8

4.2.5 FT-IR Studies

in the crystal.

The observed IR spectra of the samples at 300 K are shown in Fig. 4. 5. Normally

in the range 100-1000 cm-1, the IR bands of solids are usually assigned to vibration of

ions with significant ionic character in bonds in the crystal lattice12. The bands/shoulder

at: ~411, ~465, ~533, ~595 cm-1 are specific to the vibrations of Bi-O of various modes of

the [BiO6/2] octahedral units with the broad band centered ~ 465 cm-1 with marginal

redshift as compared with the value reported ~ 490 cm-1 13-15. As x increases in A2-A4,

more Bi3+ ions are incorporated in the lattice along with additional Al3+ ions in place of

Ni2+ ions. So, the broad peak in A2-A4 ~ 471 cm-1 is the result of the distortion of the

octahedral [BiO6/2] units due to the incorporation of additional ions. This enhances the

distortion of the [BiO6/2

] octahedral units further for the resulting red shift of the peak in

the samples A2-A4. The bands/shoulder 430, 480, 620 are assigned to the various Ni–O

45

Table 4.4.1 Unit cell dimension, a (Å), and the agreement factors after Rietveld

refinement of the unit cell structure of A1-A4 of Bi2+xAlxNi4-3xO7+δ

0.75) composite oxides.

(0.0 ≤x≤

_____________________________________________________________________ Agreement Samples Factors ___________________________________________________ A1 A2 A3 A4 _____________________________________________________________________ a (Å) 10.2678 10.1266 10.1160 10.1163

Rp

R

(%) 97.10 96.19 96.14 95.97

wp

R

(%) 99.83 96.80 96.98 96.37

exp

____________________________________________________________________

(%) 0.23 0.26 0.30 0.23

Table 4.4.2 Selected Bond lengths and bond angles of Bi2+xAlxNi4-3xO7+δ

system

________________________________________________________________________

(0.0 ≤ x ≤ 0.75)

S.No. Atoms A1 A2 A3 A4 ________________________________________________________________________ Selected Bond lengths (Ǻ): (atoms-atom-atom)

1 Bi1-O1 2.0221 (16) 1.9943 (16) 1.9922 (16) 1.9923 (16)

2 Bi2-O1 -- 4.0982 (48) 4.0940 (48) 4.0941 (48)

3 Ni1-O1 2.1367 (48) 2.1073 (48) 2.1051 (48) 2.1051 (48)

4 Ni1-O2 1.3882 (30) 1.3691 (30) 1.3677 (30) 1.3677 (30)

5 Ni2-O1 4.1554 (48) 4.0982 (48) 4.0940 (48) 4.0941(48)

6 Ni2-O2 6.5752 (24) 6.4848 (24) 6.4780 (24) 6.4782 (24)

Selected Bond angles( °): (atoms-atom-atom-atom)

7 Bi1-O1-Bi2 -- 60.8813 (1) 60.8813 (24) 60.8813 (1)

8 Bi1-O1-Ni2 60.8813 (1) 60.8813 (1) 60.8813 (24) 60.8813 (1)

9 Bi2-O1-Ni2 -- 121.7626 (1) 121.7626 (45) 121.7626 (1)

10 Ni1-O2-Ni2 128.6665 (1) 128.6665 (1) 128.6665 (20) 128.6665 (1)

11 O1-Ni2-O2 58.2540 (1) 58.2540 (1) 58.2540 (24) 58.2540 (1)

________________________________________________________________________

46

Table 4.4.3 Generated positions of atoms in the asymmetric unit of Bi2+xAlxNi4-3xO7+δ

_______________________________________________________________________

(0.0 ≤ x ≤ 0.75) system

Before Refinement After Refinement No name crystal coordinates cartesian coordinates cartesian coordinates x y z x y z x y z _______________________________________________________________________ A1 1 Bi1 0.2500 0.2500 0.2500 -2.5669 -2.5669 -2.5669 -2.5670 -2.5670 -2.5670 2 Ni1 0.7853 0.0000 0.0000 2.9293 -5.1338 -5.1338 2.9293 -5.1339 -5.1339 3 Ni2 0.2500 0.0000 0.5000 -2.5669 -5.1338 0.0000 -2.5670 -5.1339 0.0000 4 O1 0.1363 0.1363 0.1363 -3.7344 -3.7344 -3.7343 -3.7345 -3.7344 -3.7344 5 O2 0.6501 0.0000 0.0000 1.5411 -5.1338 -5.1338 1.5411 -5.1339 -5.1339 A2 1 Bi1 0.2500 0.2500 0.2500 -2.5300 -2.5299 -2.5299 -2.5317 -2.5317 -2.5317 2 Bi2 0.2500 0.5000 0.0000 -2.5300 0.0000 -5.0599 -2.5317 0.0000 -5.0633 3 Ni1 0.7853 0.0000 0.0000 2.8871 -5.0599 -5.0599 2.8890 -5.0633 -5.0633 4 Ni2 0.2500 0.0000 0.5000 -2.5300 -5.0599 0.0000 -2.5317 -5.0633 0.0000 5 O1 0.1363 0.1363 0.1363 -3.6806 -3.6806 -3.6805 -3.6831 -3.6831 -3.6830 6 O2 0.6501 0.0000 0.0000 1.5189 -5.0599 -5.0599 1.5199 -5.0633 -5.0633 A3 1 Bi1 0.2500 0.2500 0.2500 -2.5296 -2.5296 -2.5296 -2.5290 -2.5290 -2.5290 2 Bi2 0.2500 0.5000 0.0000 -2.5296 0.0000 -5.0592 -2.5290 0.0000 -5.0580 3 Ni1 0.7853 0.0000 0.0000 2.8867 -5.0592 -5.0592 2.8860 -5.0580 -5.0580 4 Ni2 0.2500 0.0000 0.5000 -2.5296 -5.0592 0.0000 -2.5290 -5.0580 0.0000 5 O1 0.1363 0.1363 0.1363 -3.6801 -3.6801 -3.6800 -3.6793 -3.6792 -3.6792 6 O2 0.6501 0.0000 0.0000 1.5187 -5.0592 -5.0592 1.5183 -5.0580 -5.0580 A4 1 Bi1 0.2500 0.2500 0.2500 -2.5284 -2.5284 -2.5284 -2.5291 -2.5291 -2.5291 2 Bi2 0.2500 0.5000 0.0000 -2.5284 0.0000 -5.0567 -2.5291 0.0000 -5.0581 3 Ni1 0.7853 0.0000 0.0000 2.8853 -5.0567 -5.0567 2.8861 -5.0582 -5.0581 4 Ni2 0.2500 0.0000 0.5000 -2.5284 -5.0567 0.0000 -2.5291 -5.0581 0.0000 5 O1 0.1363 0.1363 0.1363 -3.6783 -3.6783 -3.6782 -3.6794 -3.6793 -3.6793 6 O2 0.6501 0.0000 0.0000 1.5179 -5.0567 -5.0567 1.5184 -5.0582 -5.0581 ________________________________________________________________________

47

(a) (b)

(c) (d)

Fig. 4.4.1(a) Perspective view of the unitcell of A1 (Bi8Ni16O28), (b) A2

(Bi9Al1Ni13O28

(c) A3 (Bi

)

10Al2Ni10O28), (d) A4 (Bi11Al3Ni7O28

)

48

(a) (b)

(c) (d)

Fig. 4.4.2 (a-d) 2-dimensional view on (001) plane of A1-A4

49

(e) (f)

(g) (h)

Fig. 4.4.2 (e-h) 2-dimensional view on (111) plane of Bi2+xAlxNi4-3xO7+δ

0.25, 0.50, 0.75)

(A1-A4: x=0,

50

(a) (b)

(c) (d)

Fig. 4.4.3 (a-d) Asymmetric unit of Bi2+xAlxNi4-3xO7+δ

respectively

(A1-A4: x=0, 0.25, 0.50, 0.75)

51

(a) (b)

(c) (d) Fig. 4.4.4 (a-d) 3-dimensional electron density from (010) plane of Bi2+xAlxNi4-3xO7+δ

(A1-A4: x=0, 0.25, 0.50, 0.75) respectively

52

(a) (b)

(c) (d)

Fig. 4.4.5 2-dimensional electron density contour on (010) plane of Bi8Ni16O28 (A1-A4)

53


Fig. 4.5 FTIR spectra of samples Bi2+xAlxNi4-3xO7

crystalline system at room temperature

(A1-A4: x=0, 0.25, 0.50, 0.75) of

poly

54

stretching modes16-18. The absorption bands at: ~470, ~540, ~ 620, and ~ 828 cm-1

characteristic of the vibrations of Bi–O bonds in pyramidal [BiO3/2] units13-15. Due to the

overlapping of bands of the various modes of octahedral [BiO6/2], pyramidal [BiO3/2] and

octahedral [NiO6/2] units, the IR bands in the spectra are fairly broad. The band ~600 cm-

1 in A2-A4 is due to the Al-O bond of the octahedrally coordinated Al19. The bands at:

~721, ~ 900, and ~ 982 cm-1 are assigned to the bending, symmetric and asymmetric

stretching of Al-O-Al unit20

4.2.6 Band Structure and Density of States

, respectively.

The Figs. 4.6.1 (a)-(b) and Figs. 4.6.1 (c)-(d) show the electronic energy band

structures and the density of states, of representative samples A1 and A4, respectively,

calculated by the plane-wave density functional theory method using a commercial

version of the CASTEP programme package. This program evaluates the total energy of

periodically repeating geometries based on density-functional theory and the

pseudopotential approximation. In this case only the valence electrons of the elements are

represented explicitly in the calculations, and the valence-core interaction being

described by nonlocal pseudopotentials. The band structure of the composite oxides

predominantly having MO6 (M=transition metal) polyhedra may be represented by the

transition metal d and the O 2p orbitals9. Fig. 4.6.2 shows the calculated total density of

states and partial density of states of elements O, Ni, Al, and Bi in Bi11Al3Ni7O28 (A4:

x= 0.75). The Fermi level is pinned at 0 eV. A smearing of 0.5 eV is used to generate the

55

DOS plots. It can be seen that the samples do not have clear band gap. Four occupied

bands and one unoccupied band are visible from the density of states diagrams, in which

the bands consists of O 2s and Al 2s, Bi 6s, O 2p, Ni 4d and Al 2p orbitals in turn form

the lowest to the highest bands. The band located at -8.0 to +1.0 eV is due to O 2p

orbitals9, the band located at -8.0 to +0.5 eV is due to Bi 6s orbitals10, and the band at -

8.0 to +1.0 eV is due to the Ni 3d orbitals11. Due to the overlapping of Ni 3d and O 2p

orbitals it is reasonable to assume that Ni d orbitals will be split into t2g and eg

This means that O 2p orbitals overlap in phase with Ni t

under the

preferable octahedral field on composite oxide matrices.

2g and the Bi 6s orbitals to

form Ni-O-Bi bonds, or O 2p orbitals may overlap with Ni t2g or Bi 6s forming O-Ni-O

or Bi-O-Bi covalent bonds with partial ionic character, which comprises the valence

bands (VB). Formation of Ni-O-Bi or O-Ni-O bonds in the samples is evidenced by the

powder XRD analysis by Rietveld refinement7 of crystal structures of the samples

discussed earlier. With the incorporation of Al3+ ions partially in place of Ni3+ in the

same Wycoff sites, and the additional Bi3+ ions in different Wycoff sites in A2-A4, due to

the absence of valence Al d orbitals and with no high energy Bi orbitals above the Fermi

level, the band gap narrows down indicating the conduction band (CB) composed

predominantly of Ni eg

This results further show that the transition of semiconductor-like as in A1 to

metallic character with the Ni d band predominantly crossing the Fermi level and with no

high-energy Bi band present, in A2-A4 with the partial replacement of Ni

orbitals. This is clearly observed in the density of states plots for

the samples A2-A4 where the bands ~ 1.0 eV to 2.0 eV as observed in A1 is smeared out.

2+ ions by Al3+

ions atoms in the lattice. The band in the range -11.5 to – 8.5 eV is predominantly due to

56

Bi 4f bands and Al 2s bands, and the band in the range -20.5 to -18.0 eV consists of O 2s

band9

.

(a) (b)

(c) (d)

57

Fig. 4.6.1 (a) band structure (b) density of states of Bi2Ni4O7

density of States of Bi

(A1); (c) band structure (d)

2.75Al0.75Ni1.75O7

(A4)

58

Fig. 4.6.2 partial density of states of individual atoms of Bi, Al, Ni, O of

Bi2.75Al0.75Ni1.75O7

(A4)

4.2.7 Electron Paramagnetic Resonance Spectral Studies

59

The EPR linshapes of the samples recorded at 300 K, 77 K and 6 K are shown in

Figs. 4.7 (a-c). The lineshapes are isotropic with poorly resolved hyperfine lines. The

oxidation states of nickel ions and cationic distribution in the oxides could be studied by

EPR spectroscopy. Our chemical analysis result shows the presence of Ni2+ ions in the

samples and the concentrations are shown in Table 4.7. The Ni2+ (3d8, S=1, in strong

octahedral field) has isotopic distribution (58Ni, I= 0; 61Ni, I=3/2, natural abundance

~1.25%)21 with 58Ni:61Ni=97:1. Therefore, normally at room temperature 61

The lineshapes at 300 K show superimposition of three isotropic absorption

lineshapes due to Ni ions. The sharp peak in the middle is due to Ni

Ni ions are

not observed by EPR, however, at 77 K or at lower temperature it could be.

+ (3d9; S=1/2 in

strong octahedral field) at 58Ni (I=0) nucleus (g1iso ~ 1.993-1.994 in A1-A4, Table 4.7).

This particular signal shows decreasing intensity from A1 to A4 due to the dilution of the

magnetic ions by addition of more diamagnetic Bi3+ ions and substitution of Ni2+/Ni+ by

Al3+ ions. The very weak 4-line feature in all the four samples A1-A4 is due to the Ni+

ion at 61Ni (3d9, S=1/2, I=3/2) nucleus having similar g2iso-values (Table 4.7). The low

intensity of these lines is due to the very low relative abundances of the 61Ni nuclei,

abundances ~1.25%, as well as the very low concentration of the Ni+ ions, which

practically is the nonstoichiometry in the Ni2+

The broad and low intensity background signal is due to the Ni P

2+P at P

58PNi (3dP

8P,

S=1, I=0) nucleus at a highly distorted octahedral environment (g3RisoR, Table 4.7). It may

be mentioned that if the Ni P

2+P ion is in a highly distorted octahedron

[OR1/2R ◌۠∙∙∙NiOR4/2R∙∙∙OR1/2R] the thermal vibration of the atoms may render the geometry around

the Ni P

2+P to fluctuate as octahedron ↔ square planar at the lattice vibration frequency of

60

~10

site distribution.

12 Hz at 300 K. Since this frequency is much higher than the EPR timescale which of

~109 Hz, the distorted octahedral geometry with net spin, S=1, and I=0, renders the broad

low intensity signal, whereas at square planar configuration no signal results due to net

spin, S=02

At 77 K, since the thermal motion of the atoms decreases drastically, the

geometry around the Ni

.

2+/Ni+ ions is practically a distorted octaherdon, and the very

broad and low intensity background signal is smeared out to the Ni2+ at 58Ni (3d8, S=1,

I=0). Also at 77 K, the mI-dependent EPR linewidth due to Ni+ ions (61Ni, I=3/2; mI=-

3/2, -1/2, +1/2, +3/2) broaden due to the shorter relaxation time as compared with those

at 300 K in the adiabatic relaxation process in Ehrenfest’s sense22

As a result, the m

, where the fluctuation

in the magnetic energy levels occur at faster rate than the transition time with ΔE/ν as the

invariant (ΔE is the energy separation of magnetic levels, ν is the frequency of transition

between these two energy levels).

I-dependent linewidths broaden due to uncertainty principle, in

the case of lineshapes at 77 K as compared with those at 300 K. The observed g4iso-

values (Table 4.7) are found to be similar in A1-A4. At 6 K, the relaxation time in the

adiabatic regime becomes still relatively shorter, and as a result, the mI-dependent

linewidths increase resulting in the smearing out of the hyperfine feature at 6 K. The

broad isotopic lineshape with smeared out 4-line hyperfine feature at 6 K is due to the Ni+

at 61Ni (3d9, S=1/2, I=3/2) nucleus at a highly distorted octahedral environment (g5iso,

Table 4.7).

61

It may be mentioned here that the powder x-ray diffraction result of the same

samples shows that the unit cell type, bond lengths, and bond angles remain fairly the

same within the composition range studied. This is related to both, the microstructure and

the magnetic properties of the compounds. Thus, our observation of similar EPR giso

Table 4.7 Observed concentration of Ni

-

values within the composition range, x (0.0 – 0.75), at 300 K, 77 K, and at 6 K also

shows that the local structure and the paramagnetic site symmetry are the similar in the

samples in the range 300-6 K.

2+ ions and observed EPR giso-values of A1-A4 at

300 K, 77 K and 6 K of Bi2+xAlxNi4-3x

______________________________________________________________________

O7 (0.0 ≤ x ≤ 0.75) composite oxide system.

Sample No. of Ni2+ g1iso g2iso g3iso g4iso at 77 K g5iso

No. ions (×10 at 6 K

20

300 K /g) ________________________

______________________________________________________________________ A1 9.669 1.993 1.994 1.988 2.190 2.000 A2 8.048 1.993 1.996 1.986 2.047 - A3 6.584

A4 4.384 1.993 1.994 1.988 2.139 2.013

1.994 1.996 1.992 2.151 - ______________________________________________________________________

Table 4.8 Observed magnetization (Ms), coercivity (Hci), retentivity (Mr), total area of

the hysteresis loop, magnetic susceptibility(χ), Weiss constant(θ) and exchange integral

(j) of A1-A4 of Bi2+xAlxNi4-3x O7+δ

______________________________________________________________________________

(0.0 ≤ x ≤ 0.75) system.

S.No mass magnetization coercivity retentivity, Total magnetic Weiss Exchange (mg) Ms x 10-3 emu Hci, G Mr x10-3

(emu/G) (erg/g) x10 area susceptibility constant, integral, j

-5

______________________________________________________________________________ (μemu/g) θ (K) (eV)

A1 12.5 64.359 159.61 10.444 50.611 1.712 -146.5 0.025 A2 11.0 78.038 152.02 12.383 73.294 2.137 -207.8 0.036 A3 10.6 87.191 154.40 13.682 70.408 2.352 -230.2 0.040 A4 11.5 75.294 158.41 11.654 51.617 1.931 -217.8 0.037 _____________________________________________________________________________

62

Fig. 4.7 (a) EPR spectra at 300 K for a polycrystalline samples Bi2+xAlxNi4-3xO7+δ

A4: x=0, 0.25, 0.50, 0.75) respectively

(A1-

63

Fig. 4.7 (b) EPR spectra at 77 K for a polycrystalline samples Bi2+xAlxNi4-3xO7+δ

x=0, 0.25, 0.50, 0.75) respectively

(A1-

A4:

64

Fig. 4.7 (c) EPR spectra at 6 K for a polycrystalline samples Bi2.0Ni4.0O7+δ

Bi

(A1) and

2.5Al0.5Ni2.5O7

(A3)

65

4.2.8 Magnetic Studies

Figs. 4.8 (a)-(c) show magnetization versus magnetic field in the range -10 kG to

+ 10 kG in A1-A4, the expanded scale of magnetization in A1, and the zoom in

magnetization in the range -500 G to +500 G (inset), respectively at 300 K. The presence

of hysteresis loop in all the samples show the ferromagnetic nature of the samples at 300

K. Fig. 4(a) further shows that the magnetization is saturated up to the highest field

applied of ~ 4 kG with marginal decrease in the magnetic field for A2-A4. This result is

attributed to the incorporation of more diamagnetic Bi3+ ions and additional Al3+ ions at

the expense of Ni2+

From the Fig. 4.8 (a) we calculate the magnetic susceptibility, and show these

values along with the observed values of magnetization (Ms), coercivity (Hci), retentivity

(Mr) in Table 4.8. Using the susceptibility values we calculate the Weiss constant (θ),

using the Curie-Weiss law, which in turn is used to calculate the exchange integral (j)

ions in the crystalline lattice which affects the magnetic behaviour of

the samples as observed.

23,

and the values are presented in Table 4.8. The relatively low values of values of the

coercivity, retentivity, magnetic susceptibility ~ 10-5

The coercitivity is non-symmetric (Fig. 4.8 (c)) implies a non-homogeneous

domain magnetic structure which leads us to infer that the behaviour of Ms in the

samples is influenced largely by microstructural factors

emu/gG with saturation magnetic

field ~ 4 kG shows fairly waek ferromagnetic nature of the samples. The negative values

of Weiss constant shows that the magnetic behavior is ferromagnetic in nature.

24. It may be mentioned that the

66

samples have certain porocity, and these strongly influence domain-wall motion and

hence the magnetic properties.

Fig. 4.8. (a) Magnetic hysteresis loops of A1-A4 at 300 K . This show the soft ferro

magnetic nature. (b) shows an expanded scale of the nominal composition

x=0 (A1). (c) The upper left inset shows the zoom in the low applied field

regime.

67

The behaviour of Hci is influenced by two competing microstructural features: grain size

and pores25

4.2.9 Optical Absorption Studies

. As we have mentioned above that the microstructure of the samples having

pores and no regular variation in grain size.

The DRS spectra of A1-A4 shown in Fig. 4.9 (a). consists of a absorption bands

around 380, 420, 460, 640 and 720 nm, which is characteristic of octahedral Ni

environment. The strong absorption over the whole wavelength range has been attributed

to intervalence absorption in the system which contains some Ni3+ 26. The peaks in the

region 350-380 nm, are attributed to the 3A2g →1T1g and 3A2g → 3T1g transitions. The

absorption at 420 nm is attributed to 3A2g (F) →3A2g (P), 460 nm to 3T1g → 1T2g, 640 nm

to 3A2g (F) → 3T1g(F), and 720 nm to 3A2g (F) – 3T1g (P) 27. However, this assignment is

in conflict with detailed energy diagrams28. The transitions that involve a change in spin

multiplicity are in general much less intense, viz. 25% or less of the transition to the 3A2g

→3T1g

The direct band gap (E

. This result is in conformity with our IR absorption studies in terms of local

geometries and site symmetries.

g) values (Table 4.9) of A1-A4 were evaluated from the

plot of squares of optical absorption coefficient (αhν)2 as function of the photon energy

(hν) shown in Fig. 4.9 (b). The absorption coefficient obeys the following equation:

∝=𝐶𝐶ℎ𝜈𝜈

�ℎ𝜈𝜈 − 𝐸𝐸𝑔𝑔�12

68

Where h is the Planck constant, ν is the radiation frequency, Eg

The band gap values calculated by band structure using castep are lower than the

band gap energy values by optical absorption studies. This underestimation is mainly due

to the well known shortcoming of exchange correlation functions. However, as a kind of

effective approximation its relative calculation value was quite exact, and it did not affect

theoretical analysis on electronic structure analysis.

is the band gap

energy and C is a constant.

Table 4.9. Direct band gap values of A1-A4 calculated using absorption coefficient.

______________________________________ Sample code Band gap (eV) ______________________________________ A1 2.47 A2 2.30 A3 2.34 A4 2.53 ______________________________________

69

Fig. 4.9 (a). UV – vis DRS spectra of compositions Bi2+xAlxNi4-3xO7+δ

x=0, 0.25, 0.50, 0.75) respectively

(A1-A4:

Fig. 4.9 (b) Band gap values of A1-A4 obtained by plotting (αhν)2

vs. hν

1 2 3 4 5-1

0

1

2

3

4

5

(αhν

)2 X

1010

(cm

-1 e

V 2 )

hν (eV)

A1 A2 A3 A4

70

4.3 Conclusion

In conclusion synthesis the new series of Bi2+xAlxNi4-3xO7 (0.0 ≤ x ≤ 0.75) via

sol-gel method is done and studied its structure and physical properties. Analysis of the

powder XRD patterns by Fullprof shows cubic unit cell with lattice parameters in A1-A4:

a = 10.2678, 10.1197, 10.1183, 10.1134 Å and space group Pm3n. From the comparison

of the observed (A1-A4: 4.398, 4.946, 5.169, 4.825 g/cm3) and calculated (A1-A4: 4.680,

4.999, 5.097, 4.520 g/cm3) densities, the Z value is determined to be 4. The average

crystallite sizes in A1-A4 determined by Scherrer’s relation are found to be in the range ~

42-61 nm. The DTA-TG results show no phase transitions in the range 25-800 ºC. SEM

micrographs show that as the concentration of aluminium increases the shape of the

particles changes from globular shape to quadrupods-like shape in A1 to A4. Rietveld

refinement of the unit cell structure developed on space group Pm3n shows appreciably


96.37 %; Rexp = 0.23, 0.26, 0.30, 0.23 % in A1-A4, respectively. The Bi-O and Ni-O

bonds of different sites show marginal variations in the samples. Circular nature of the

Fourier electron density 2D-contours around Ni and O due to valence orbitals shows the

partial ionic character of Ni-O bonds. The IR spectral results at 300 K show Ni-O (~ 411

cm-1), Bi-O (~441 cm-1) and Bi-O (471 cm-1) in [BiO6/2

The DFT calculations on unit cell structure of A1-A4 are done for electronic

energy band structures and density of states (DOS) using a CASTEP programme

package. This result shows the transition of semiconductor-like as in A1 to metallic

character with the Ni e

] octahedral units.

g band predominantly crossing the Fermi level, in A2-A4. The

presence of hysteresis loop with very low values of the coercivity, retentivity, magnetic

71

susceptibility ~10-5 emu/gG with saturation magnetic field ~ 4 kG shows fairly weak

ferromagnetic nature of the samples at 300 K. The observed X-band EMR lineshapes at

300, 77 and 6 K are isotropic in nature. The similar giso-values in A1-A4 in the range

300-6 K show the similar local structure and the paramagnetic site symmetry in the

samples. The lineshapes at 300 K show decreasing intensity from A1 to A4 due to the

dilution of the magnetic ions by addition of more diamagnetic Bi3+ ions and substitution

of Ni2+ (3d8, S=1, in strong octahedral field) by Al3+ ions. The very weak 4-line feature in

A1-A4 is due to the Ni+ (58Ni, I= 0; 61Ni, I=3/2, natural abundance ~1.25%) ions. The mI-

dependent EMR linewidths due to Ni+ ions broaden due to the shorter relaxation time at

77 K. The broad isotropic lineshape at 6 K is also predominantly due to the Ni+ ions.

Diffuse reflectance spectra (DRS) at 300 K show absorption at 420 nm due to 3A2g (F)

→3A2g (P), 460 nm due to 3T1g (F) → 1T2g (D), 640 nm due to 3A2g (F) → 3T1g (F), and at

720 nm due to 3A2g (F) – 3T1g

(P) transitions. The theoretical band gap energy values are

compared with the band gap values by optical absorption studies.

72

References

[1] R.A. Roy and R. Roy, Mat. Res. Bull. 19 (1984) 169.

[2] S. Komarneni, E. Breval, and R. Roy, J.Mat. Sci. 21 (1986) 737.

[3] T.K. Ghorai, D. Dhak, S. Dalai, and P. Pramanik, J. Alloys Compd. 463 (2008) 390.

[4] F. J. Morin, Phys. Rev. B. 93 (1954) 1199.

[5] J. R. Carjaval, FULLPROF program, Rietveld Pattern Matching Analysis of Powder

Patterns, ILL, Grenoble, 1990.

[6] H.P. Klug and L.E. Alexander, “X-ray Diffraction procedures for polycrystalline and

amorphous Materials”, (2nd ed.) (John Wiley & Sons, New York, 1997).

[7] L.B. McCusker, R.B.V. Dreele, D.E. Cox, D. Louër and P. Scardi, J. Appl. Cryst. 32

(1999) 36.

[8] J. Hugel and C. Carabatos, J. Phys. C: Solid State Phys., 16, (1983) 6713.

[9] D. Li, J. Zheng, and Z. Zou, J. Phys. Chem. Solids, 67, (2006) 801.

[10] J. Zhou, Z. Zou, A.K. Ray and X.S. Zhao, Ind. Eng. Chem. Res., 46, (2007) 745.

[11] H. Wei, J.J. Liang, B.Z. Sun, Q. Zheng, X.F. Sun, P. Peng, M.S. Dargusch, and X.

Yao, Philos. Mag. Letts., iFirst, (2010) 1. .

[12] V.A.M. Brabers, Phys. Stat. Sol. 33 (1969) 563.

[13] V. Dimitrov, Y. Dimitriev and A Montenero J Non–Cryst Solids 180 (1994) 51.

[14] R. Iordanova, Y. Dimitriev, V. Dimitrov, S. Kassabov and D. Klissurski, J. Non-

Cryst. Solids 204 (1996) 141.

[15] I. Ardelean, S. Cora and D. Rusu Physica B 403 (2008) 3682.

[16] S. Mochizuki, Phys. Status Solidi B 126 (1984) 105.

[17] S. Mochizuki, Phys. Status Solidi B, 110 (1982) 219.

73

[18] C. Xua, K. Honga, S. Liua, G. Wanga and X. Zhaob J. Cryst. Growth 255 (2003)

308.

[19] I.H. Joe, A.K. Vasudevan, G. Aruldhas, A.D. Damodaran and K.G.K. Warrier

J. Sol. Stat. Chem. 131 (1997)181.

[20] K. Nakamoto, “Infrared and Raman Spectra of Inorganic and Coordination

Compounds”

[21] A. Abragam and B. Bleaney, “Electron Paramagnetc Resonance

(Wiley Interscience, New York, 1986)

of Transition Ions”

(

,

Oxford, England: Oxford University Press, 1970)

[22] L. Navaro and E. Pérez, Arch. Hist. Exact Sci., 60 (2006) 209.

[23] C.P. Poole “Electron Spin Resonance”

[24] D. Jiles, “Introduction to Magnetism and Magnetic Materials

, (Interscience, NewYork, 1967).

”

UK, 1991).

, (Chapman & Hall,

[25] M. Al-Haj J. Magn. Magn. Mater. 311(2007) 517.

[26] B. Scheffer, J.J. Heijeinga and J.A. Moulijn J. Phys. Chem. 91 (1987) 18.

[27] K.B.N. Sarma, B.J. Reddy and S.V.J. Lakshman, Phys. Scr, 28 (1983) 125.

[28] C.J. Ballhausen, “Introduction to Ligand Field Theory”, (McGraw-Hill, New

York, 1968)

74

CHAPTER – V

Structure-Property Relations in New Bi2+xCexNi4-3xO7+δ

5.1.Introduction

(0.25 ≤ x ≤ 1.0) Series

The current increasing interest in designing and preparing rare-earth - transition

metal oxides have received extensive attention due to their unique 3d-4f orbital

interactions, structure properties and fascinating chemical and physical properties such as

conductivity and magnetic ordering that result from two distinct energy level orbitals (3d

of transition metal and 4f of rare-earth ions)1

The proposed model in which the 4f electrons of the rare earth ions polarized their

5d bands, which give the 5d-3d short range interaction with the transition metal

. Furthermore, the systematic study of a

family 3d-4f compounds can lead to insights regarding the contributions to the overall

magnetic behavior provided by a given lanthanide ion.

2. The 5d

band polarizations are parallely oriented to 4f moments because of the local 4f–5d

exchange interactions are positive3

5.2. Results and Discussion

.

5.2.1. Powder X-ray diffraction Studies

The samples are prepared by sol-gel method via nitrate-citrate route as discussed in

chapter III. Fig. 5.1 shows the room temperature powder X-ray diffraction patterns for

the B1-B4 samples. Powder XRD data were collected for the four samples with x= 0.25,

0.50, 0.75, and 1.0 and analyzed by Rietveld refinement using the program Fullprof

software package. No changes on the lattice parameters of the cubic phase were found.

75

Introduction of Ce in to the unit cell do not show any changes in the lattice parameters

(Table 5.1). The refinement was done in space group Pa-3.

The density values are calculated using XRD results and compared with

experimental density value are 5.364, 4.883, 4.757, 4.854 g/cc in B1-B4 respectively.

The average crystallite sizes of the samples of B1-B4 are found to be in the range 32-51

nm. The average crystallite sizes in the samples are determined by Scherrer’s relation

and the values are also shown in Table 5.1. The crystallite size is decreased from 51 to

32 nm. This is attributed to the substitution of cerium by nickel. The larger cerium ions

resulted in the formation of smaller nanorods than did the aluminium ion under identical

composition and reaction conditions. These results further confirm that the size and

dipole polarizability of the lanthanide ion dominate the size of the new series

Bi2+xCexNi4-3xOs


(0.25 ≤ x ≤ 1.0).

sizes in B1-B4 of Bi2+xCexNi4-3xO7+δ

Unit cell data/Sample B1 B2 B3 B4


________________________________________________________________________

________________________________________________________________________ Cell Formula Bi9Ce1Ni13O28 Bi10Ce2 Ni10 O28 Bi11Ce3Ni7O28

Bi12Ce4Ni4O28

Lattice parameter* a (Å) 10.7969 10.8036 10.7946 10.8182

Cell volume (Å3

Space group Pa-3 Pa-3 Pa-3 Pa-3

) 1258.64 1260.85 1257.83 1266.10

Z 4 4 4 4

Calculated density (g/cc) 5.714 4.485 4.425 4.920

Experimental density (g/cc) 5.364 4.883 4.757 4.854

Average crystallite size (nm) 51 46 47 32

________________________________________________________________________

76

* before refinement

Fig. 5.1. Powder XRD- pattern of Bi2+xCexNi4-3xO7+δ

(x = 0.25, 0.50, 0.75, 1.0) (B1-B4)

77

5.2.2 SEM and EDX

The scanning electron microscope (SEM) images of the samples are shown in

Figs. 5.2.1 (a) – 5.2.4 (a). SEM micrograph shows nanorod-like structure in

Bi3.0Ce1.0Ni1.0O7

EDAX was used to check the distribution of individual elements within the

grains. The EDAX analysis indicated a homogeneous distribution of all elements, within

the grains of the samples. The compositions of the samples were characterized by EDAX

given in Fig. 5.2.1 (b) – 5.2.4 (b). EDAX results show that all samples had the

stoichiometry close to nominal. The mapping of all samples confirms the high dispersion

and homogeneity.

. The nanorods present in the sample are irregular shape and size.

Investigation of various regions of the sample gave similar results with respect to the

particle size and shape.

5.2.3 Thermal studies

Fig. 5.3 shows the TG/DT/DSC traces. The weight loss upto 400 ºC corresponds

to removal of coordinated and trapped water. From 400 – 600 ºC, the slight weight loss

may be the removal of O2

or the reduction of Ni ions present in the samples. There is no

significant thermal changes in all the samples. Because it is determined that the

incorporation of doped ions of lanthanides contributes to a growth of the thermal stability

of these samples.

78

(a)

(b)


Bi2.25Ce0.25Ni3.25O7+δ

selected grain.

(B1) with compositions of the elements in randomly

79

(a)

(b)



80

(a)

(b)



81

(a)

(b)



82

Fig. 5.3. TGA/DTA/DSC traces of Bi2+xCexNi4-3xO7+δ

(x = 0.25, 0.50, 0.75, 1.0) (B1-B4)

83

5.2.4 Crystal Structure Refinement and Electron Density Fourier Mapping

The crystal structure of all the samples shown in Fig. 5.4.1 (a-d) are developed

with space group Pa-3, z = 4 with wycoff positions, Bi3+ (1)- 8(c), Ni2+ (1) – 8(c), Ni2+ (2)

– 8(c), O2-(1) – 24(d), O2-

The Tables 5.2, 5.3 (a-d), 5.4 (a-d) also show the refined atomic position

coordinates, bond lengths and bond angles in the asymmetric units of B1-B4,

respectively. Bond lengths and bond angles are calculated. Fourier maps of electron

density and iso-electronic surfaces are exhibited the electron density of atoms and the

structure of the unit cell.

(2) – 4(a). The refinement has done on the structures show

good agreement factors with R-values Rp = 98.57 %, Rwp = 96.55 %, Rexp = 0.18 %.

No significant changes are observed in the bond lengths and bond angles

suggesting that the total bonding interaction is practically constant in the composition

range studied. This is related to the microstructure of the samples which show uniform

behaviour. 2-dimensional electron density contour on (001) and (111) plane are shown in

Fig. 5.4.2 (a-c)-5.4.5 (a-c). The Fourier mapping of the 3-dimensional electron density

from xz-plane (010), and the 2-dimensional electron density contour on (010) plane are

shown in Fig. 5.4.6 (a-d) and Fig. 5.4.7 (a-d).respectively. The circular nature of the

contours around the Ni and O due to localized valence orbitals and in the lattice sites with

isolated overlapping contours also shows the partial ionic character of Ni-O bonds4

in the

crystal, no significant difference is occurred in all charge density maps of B1-B4, due to

the Pa-3 group symmetry.

84

(a) (b)

(c) (d)

Fig. 5.4.1 (a-d) Perspective view of the unitcell of Bi2+xCexNi4-3xO7+δ

0.75, 1.0) B1-B4

(x = 0.25, 0.50,

85

(a) (b)

(c)


86

(a)

(b)

Fig. 5.4.3 (a) 001 plane; (b) 111 plane of B2

87

(a) (b)

(c)


88

(a) (b)

(c)


89

(a) (b)

(c) (d)

Fig. 5.4.6 3-dimensional electron density from (010) plane of (c) B3; (d) B4

90

(a) (b)

(c) (d)

Fig. 5.4.7 2-dimensional electron density contour on (010) plane of (a-d) Bi2+xCexNi4-3x

O7+δ

and O atoms on the plane.

(x = 0.25, 0.50, 0.75, 1.0) B1-B4 showing the symmetric positions of Ni

bi3+_3

bi3+_4

ce3+

ce3+_2

o2-.

o2-._2

0 .1 .2 .3 .4 .5 .6 .7 .8 .9 1

0

.1

.2

.3

.4

.5

.6

.7

.8

.9

1

x/a

y/b

bi3+_3

bi3+_4

o2-.

o2-._2

Pk_9

Pk_9_2

Pk_9_13

Pk_9_14

0 .1 .2 .3 .4 .5 .6 .7 .8 .9 1

0

.1

.2

.3

.4

.5

.6

.7

.8

.9

1

x/a

y/b

91

Table 5.2 Generated Cartesian coordinates (xyz) before and after refinement of the atomic positions in the asymmetric unit in Bi2+xCexNi4-3xO7+δ

0.75, 1.0) B1-B4.

(x = 0.25, 0.50,

_________________________________________________________________ sample Before Refinement After Refinement code No. name cartesian coordinates cartesian coordinates x y z x y z _________________________________________________________________ B1 1 Bi1 -2.6376 -2.6376 -2.6376 -2.6371 -2.6371 -2.6371

2 Bi2 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 3 Ni1 -1.6680 -1.6680 -1.6680 -1.6677 -1.6676 -1.6676 4 Ni2 4.1333 4.1332 4.1332 4.1324 4.1324 4.1323

5 O1 3.3986 1.7629 -4.4731 3.3979 1.7626 -4.4721 6 O2 -4.8972 -4.8972 -4.8972 -4.8962 -4.8962 -4.8961 B2 1 Bi1 2.9095 2.9094 2.9094 2.9096 2.9096 2.9095 2 Bi2 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 3 Ni1 1.8399 1.8399 1.8399 1.8400 1.8400 1.8399 4 Ni2 -4.5592 -4.5592 -4.5591 -4.5594 -4.5594 -4.5593 5 O1 3.7488 1.9446 -4.9340 3.7490 1.9447 -4.9342 6 O2 -5.4019 -5.4018 -5.4018 -5.4021 -5.4021 -5.4021 B3 1 Bi1 -2.9070 -2.9070 -2.9070 -2.9074 -2.9073 -2.9073 2 Bi2 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 3 Ni1 -1.8384 -1.8383 -1.8383 -1.8386 -1.8385 -1.8385 4 Ni2 4.5554 4.5554 4.5553 4.5559 4.5559 4.5558 5 O1 3.7457 1.9430 -4.9299 3.7461 1.9432 -4.9304 6 O2 -5.3974 -5.3973 -5.3973 -5.3980 -5.3979 -5.3979 B4 1 Bi1 -2.9134 -2.9134 -2.9133 -2.9161 -2.9161 -2.9161 2 Bi2 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 3 Ni1 -1.8424 -1.8424 -1.8423 -1.8441 -1.8441 -1.8441 4 Ni2 4.5654 4.5653 4.5653 4.5697 4.5696 4.5696 5 O1 3.7539 1.9472 -4.9407 3.7574 1.9491 -4.9453 6 O2 -5.4092 -5.4091 -5.4091 -5.4143 -5.4142 -5.4142 __________________________________________________________________

92

Table 5.3 (a) Selected bond length of B1

_____________________________________

S. No. Atom1 Atom2 distance quant _____________________________________ 1 Bi1 Ni1 1.6791 8

2 Bi1 O1 2.3923 24

3 Bi2 Ni1 2.8884 8

4 Bi2 O1 2.3519 24

5 Ni1 O1 3.7956 6

6 Ni1 O1 2.5864 24

7 Ni2 O1 3.9101 24

8 Ni2 O2 1.3230 8

9 O1 O2 3.4991 24

10 Ni2 O1 2.7506 12

____________________________________

Table 5.3 (b) Selected bond length of B2 _______________________________ No atom1 atom2 distance quant ______________________________ 1 Bi1 Ni1 1.8526 8

3 Bi1 O1 2.6395 24

4 Bi2 O1 2.5949 24

5 Ni1 O1 2.8536 24

6 Ni2 O2 1.4596 8

7 O1 O2 3.8606 24

8 Ni2 O1 3.0348 12

_______________________________

93

Table 5.3 (c) Selected bond length of B3 __________________________________ No atom1 atom2 distance quant __________________________________ 1 Bi1 Ni1 1.8512 8

2 Bi1 O2 4.3139 8

3 Bi1 O1 2.6374 24

5 Bi2 O1 2.5929 24 6 Ni1 O2 6.1651 8 7 Ni1 O1 4.1846 6 8 Ni1 O1 2.8514 24 9 Ni2 O1 9.8727 12 10 Ni2 O2 1.4585 8 11 Ni2 O1 3.0325 6 __________________________________ Table 5.3 (d) Selected bond length of B4 _______________________________ No atom1 atom2 distance quant _______________________________ 1 Bi1 Ni1 1.8568 8 2 Bi1 O1 2.6454 24 3 Bi2 O1 2.6007 24 4 Ni1 O1 2.8600 24 5 Ni1 O1 4.0391 12 6 Ni2 O1 4.3238 24 7 Ni2 O2 1.4629 8 _______________________________

Table 5.4 (a) Selected bond angle of B1 _____________________________________ No atom1 atom2 atom3 angle quant _____________________________________ 1 Ni1 Bi1 O2 180.0000 1 2 Bi1 Ni1 Bi2 180.0000 1 3 Ni1 Bi2 O1 93.8714 1 4 Bi2 O1 Ni2 52.8869 1 _____________________________________

94

Table 5.4 (b) Selected bond angle of B2 _______________________________________ No atom1 atom2 atom3 angle quant _______________________________________ 1 Ni1 Bi1 O2 179.9693 1 2 Bi2 O1 Ni2 52.8869 1 ______________________________________

Table 5.4 (c) Selected bond angle of B3

______________________________________

No atom1 atom2 atom3 angle quant

______________________________________

1 Ni1 Bi1 O2 179.9901 2 2 Ni1 Bi1 O1 76.6644 18 3 O2 Bi1 O1 103.3357 18 4 Bi1 O1 Ni2 107.7948 12 5 Bi1 O2 Ni2 180.0000 6 ______________________________________ Table 5.4 (d) selected bond angle of B4 ____________________________________ No atom1 atom2 atom3 angle quant ____________________________________ 1 Ni1 Bi1 O1 76.6642 18 2 Bi1 Ni1 O1 64.1593 12 3 O1 Bi1 O1 114.8470 12 4 Bi1 O1 Ni1 39.1766 6 5 Bi1 O1 Ni2 40.1287 12 6 Bi1 O1 Bi2 121.6624 6 ____________________________________

5.2.5 Band structure and density of states calculations

Band structure Fig. 5.5.1 (a, c, e, g) and density of states Fig. 5.5.1 (b, d, f, h)

were calculated by castep code. The band tails crossing the Fermi energy levels. The

band gap is ~0.009 eV for all samples. The calculated total densities of states (TDOS)

and the partial densities of states (PDOS) from Bi, Ce, Ni and O are shown for B4. The

95

basis set included Bi 6s; 6p orbitals, Ce 6s; 6p; 5d and 4f orbitals, Ni 4s; 4p and 3d

orbitals and O 2s and 2p orbitals.

Between -6 and 1 eV, the DOS are mostly from Ni-3d, Ce-4f, Ce-5d, Bi-6p and O

2p orbitals with the strong peak just below Fermi level due to Ni-3d orbitals5. The 4f

electrons of the rare earth ions polarize their 5d bands2

This is unusual considering that Ce-4f orbitals are not often considered as

involved in chemical bonding, but as localized states. Moreover, integrating the Ce-4f

PDOS indicates a small Ce-4f contribution to the occupied states of these compounds.

Surprisingly, the 5p states of Ce also provide a significant contribution to valence band.

, which give the 5d-3d short range

interaction with the transition metal. The exact position and dispersion of these orbitals

depends on the composition for the four compounds, this band is centered at 2.5 eV

below the Fermi level. Just below the Fermi level in all cases, the large peak assigned to

Ni-3d orbitals also contains contributions from Ce-5d, Bi-6p and O-2p levels, which

suggests interactions between these orbitals of the conduction band and the Ce-4f

orbitals.

The large peak assigned to Ce-5d and 6p orbitals also contains contributions from

Bi 6s and Ni-3d levels, which suggests interactions between these orbitals of the

conduction band. This is usual considering that Ce-4f orbitals are not often considered as

involved in chemical bonding, but as localized states. Moreover, integrating the Ce-4f

PDOS indicates a small Ce-4f contribution to the occupied states of these compounds6

.

96

(a) (b)

(c) (d)

Fig. 5.5.1 (a) band structure (b) density of states of B1; (c) band structure (d) density of

states of B2

97

(e) (f)

(g) (h)

Fig. 5.5.1 (e) Band structure (f) density of states of B3; (g) band structure (h) density of

states of B4

98

Fig. 5.5.2 (a) Total density of states (b-e) Partial density of states of individual atoms Bi,

Ce, Ni and O of B4

99

The valence bands lying about between -19 and -16 eV, -12 and -9, and between

about -6 and 1 eV are almost contributions from oxygen 2p states. The DOS is higher for

O 2p states (the largest DOS of 30 electrons/eV) than Bi 6p states (the largest DOS of 15

electrons/eV)7

A remarkable character in the DOS is that there is a narrow band centered below

the E

. It means that some of the electrons from Bi 6p transform into the valence

bands and take part in the covalence interactions between Bi and O atoms.

F

5.2.6 Electron Paramagnetic Resonance Spectral Studies

which can be viewed as the contribution of Ni atoms because the projected DOS

of Ni atoms fills all of the total DOS narrow band. Since the Ni atoms have unpaired 3d

electrons which are localized in octahedral lattice, it is possible that the localized electron

configuration is one of the factors of the observed weak ferromagnetic behavior in the M-

H curve.

The observed X-band electron paramagnetic resonance (EPR) line shapes at 300

K (Fig. 5.6 (a)) are isotropic except B2. EPR spectra show the presence of two g-values

for B2 (x =0.5) g║ = 2.429, g┴ = 2.081 and g = 2.003, g = 2.065, 2.003 due to formation

of two different sites and 2.002 for B1, B3 and B4 (x =0.25, 0.75 and 1.0) respectively.

The marginal decrease in g-values by replacement of Ni2+ by Ce3+ and Bi3+ in B1-B4

indicates increase in covalence nature of the Ni–O bond in the samples. EPR spectra

recorded at 77 K (Fig. 5.6 (b)) show two g-values g1 and g2 which correspond to Ce3+

with poorly resolved 6-line feature and Ni2+

The EPR signal is due to the Ni

ions, respectively.

2+ (3d8, S=1) ion in strong octahedral field (58Ni,

I=0, NA ~97%) nuclei with superimposed efffect from (61Ni, I=3/2, ~1.37%)8 nuclei. Ni+

ion signal is not observed due to low concentration. The marginal decrease in g-values

by replacement of Ni2+ by Ce3+ and Bi3+ in B1-B4 indicates increase in covalency of the

100

Ni–O bond in the samples. Cerium has no magnetic isotopes (4f1, Ce, I=5/2) and the

EPR spectrum contains no hyperfine structure which could facilitate the detection and

identification of the different types of Ce3+ sites. All the EPR signals reported here are

due to single cerium ions that are not coupled by exchange interaction with other Ce3+

ions. Ce3+ signal is not observed at 300 K. Observed changes in the EPR line width can

be related to magnetic dilution of Ni2+ spin systems caused by incorporation of non-

magnetic ion (Bi3+ ions). EPR spectra recorded at 77 K show two g-values g1 and g2

which correspond to Ce3+ and Ni2+ ions, respectively.

5.2.7 Magnetic Studies

From Fig. 5.7, it is observed that weak hysteresis loop show the soft

ferromagnetic behavior at room temperature. Hysteresis loops do not appreciably change

with x, it is inferred that any possible concentration of rare-metal contribution is

negligible. From this result, it is also evidenced that there are no substantial changes in

coercivity values from x=0.5 to x=1.0. The coercivity and retentivity values are

remaining the same order of magnitude, which indicates that the magnetic behavior of all

samples were uniform. When compared the magnetization of A1-A4, the magnetization

of B1-B4 is higher in one order of magnitude due to the substitution of magnetic rare-

earth ion in B1-B4.

5.2.8 Optical Absorption Studies

Fig.5.8 (a). Shows the Diffuse reflectance spectra of B1-B4. In region 350-380

nm peaks can be seen which are attributed to the 3A2g →1T1g and 3A2g →3T1g

transitions9. Alternatively, absorption at 460 nm has been ascribed to a transition to 3T1g,

415 nm to1A1g and 377 nm to 1T1g. However, this assignment is in conflict with detailed

101

energy diagrams. The observed bands 280, 300 and 352 nm were assigned to be the

transition to the (4f1) 2 F5/2-(5d1) →2T2g of Ce3+ 10

The direct band gap (E

.

g) values (Table 5.8) of B1-B4 were evaluated from the

plot of squares of optical absorption coefficient (αhν)2

Table 5.7. Observed magnetization (M

as function of the photon energy

(hν). As the concentration of Ce increases the band gap energy also increases and the

band gap widening is shown in Fig 5.8 (b). The theoretical band gap values calculated

from band structure using DFT calculations are lower than the band gap energy values by

optical absorbtion studies and this underestimation is mainly due to the well known

limitation of exchange correlation functions.

s), coercivity (Hci), retentivity (Mr


), total area of

2+xCexNi4-3xO7+δ

≤1.25) system.

(0.25 ≤ x

________________________________________________________________________ S.No mass magnetization coercivity retentivity, Mr Total magnetic (mg) Ms X 10-2 emu Hci, G X10-3

(erg/g) X10 (emu/G) area susceptibility

-5

________________________________________________________________________ (emu/g)

B1 37.5 19.333 406.36 13.262 19.574 2.682 B2 38.3 11.556 111.56 5.091 14.669 2.089 B3 33.7 11.530 122.33 6.236 38.887 7.813 B4 30.4 12.699 120.38 7.020 49.434 2.466 ________________________________________________________________________

Table 5.8 Band gap energy values of B1-B4 respectively

___________________________________ Sample code Band gap energy (eV) ___________________________________

B1 2.13 B2 2.51 B3 2.58 B4 2.64

___________________________________

102

(a)

Fig. 5.6 (a) EPR spectra of Bi2+xCexNi4-3xO7+δ

(x= 0.25, 0.50, 0.75, 1.0) (B1-B4) at 300

K

103

(b)

Fig. 5.6 (b) EPR spectra of Bi2+xCexNi4-3xO7+δ

(x = 0.25, 0.50, 0.75, 1.0) (B1-B4) at 77

K

104

Fig. 5.7 M vs H plot showing weak hysteresis loop for Bi2+xCexNi4-3xO7+δ

0.75, 1.0) B1-B4

(x = 0.25, 0.50,

105

Fig. 5.8 (a) Optical absorption spectra of Bi2+xCexNi4-3xO7+δ (x = 0.25, 0.50, 0.75, 1.0)

(B1-B4)

106

Fig 5.8 (b) Variation of (αhν)2

calculated extrapolating the curve.

as a function of energy (hν) of B1-B4. Band gap energy is

5.3 Conclusion

This chapter concludes with the synthesis of single phase composite

oxides with cubic unit cell in Bi2+xCexNi4-3xO7 (B1-B4: x = 0.25, 0.50, 0.75, 1.0) system

is obtained by sol-gel method via nitrate-citrate route and correlating the results with that

of Chapter- 4. Instead of non-magnetic aluminium ions as in system- A, magnetic rare-

earth cerium Ce3+ (4f1) ions are substituted and the variations/similarities in properties are

discussed. Unit lattice parameters in B1-B4 are: a = 10.7969, 10.8036, 10.7946, 10.8182

Å; Z= 4 and space group Pa-3. The average crystallite sizes in B1-B4 determined by

Scherrer’s relation are found to be in the range ~ 32-51 nm. Thermogravimetric (TG)

results in the range 50-1000 ºC indicated that the weight loss up to 400 ºC is due to

gradual removal of moisture present in the samples and the weight loss around 600 ºC

with an endothermic peak in DSC/DTA traces is due to reduction of Ni2+ to Ni+

1 2 3

0

1

2

(αhν

)2 X

1010

(cm

-1 e

V 2 )

hν (eV)

B1 B2 B3 B4

with

107

consequent removal of O2. SEM micrographs show that formation of the rod-like shape

increases from B1 to B4. The EDX show stoichiometry close to nominal in the samples.

The unit cell structure of the samples are developed with space group Pa-3, Z = 4 with

Wycoff positions, Bi3+ (1) in 8(c), Ni2+ (1) in 8(c), Ni2+ (2) in 8(c), O2-(1) in 24(d) and O2-

(2) in 4(a). The Rietveld refinement on the unit cell structures show agreement factors

(R-values) Rp = 98.57 %, Rwp = 96.55 %, Rexp = 0.18 % for the representative sample

B1. Calculated bond lengths Bi1-O1: 2.3923 Å, Ni1- O1: 2.5864 Å, Ni2-O1: 2.7506 Å

show that two different Ni ions present in all the samples. Circular nature of Fourier

electron density contours shows the partial ionic nature of Ni-O bonds in the samples.

The FT-IR spectroscopic results show the presence of Ni-O, Bi-O and Ce-O bonds in the

samples. The magnetic moment versus magnetic field plots show hysteresis behavior

with low values of coercivity and retentivity indicating the weak ferromagnetic nature of

the samples as observed in the case of A1-A4. The band structure and density of states

show the metallic character with the Ni 3d band predominantly crossing the Fermi level

and with no high-energy Bi or Ce band present. The energy band gap, Eg, is found to be

~ 0.01 eV in B1-B4. The observed X-band EMR lineshapes at 300 K are isotropic except

in B2. The EMR spectra show the presence of two g-values for B2 (x =0.5) g║ = 2.429, g┴

= 2.081 and g = 2.003, due to formation of two different Ni2+ (58Ni, I=0, NA ~97%; 61Ni,

I=3/2, ~1.37%) sites and giso = 2.065, 2.003, 2.002 for B1, B3 and B4 (x =0.25, 0.75 and

1.0) respectively. The EMR lineshapes at 77 K also show two isotropic g-values g1=

2.852, 2.836, 2.836, 2.836 which correspond to Ce3+(4f1) in B1-B4 and poorly resolved

6-line feature and g2 = 2.018, 2.009, 2.009, 2.026 with poorly resolved 4-line feature due

to Ni2+ ions in B1-B4, respectively. The marginal decrease in g-values is due to the

108

addition of diamagnetic Bi3+ (6s2) ions in the matrix in place of Ni ions which partially

distorts the site symmetry of the Ni ions in B1-B4. The optical absorption studies show

the 3A2g →3T1g (350 -380 nm), 3A2g → 1A1g (415 nm), 3A2g →1T1g (460 nm), 2F5/2 →

2T2g (4f1→5d1)) (280-350), 3A2g (F) → 3T1g (P) (720 nm) transitions in the samples and

the band gap energy values calculated by plotting (αhν)2

vs. hν are compared with

theoretical band gaps by DFT calculations.

References

[1] E. Burzo, L. Chioncel, R. Tetean and O. Isnard, J. Phys.: Condens. Matter. 23 (2011)

026001.

[2] E. Burzo and P. Vlaic, J. Magn. Magn. Mater. 290-291 (2005) 599.

[3] E. Burzo and L. Chioncel, J. Optoelectron. Adv. Mater. 6 (2004) 917.

[4] J. Hugel and C. Carabatos, J. Phys. C: Solid State Phys., 16 (1983) 6713.

[5] H. Wei, J. J. Liang, B. Z. Sun, Q. Zheng, X. F. Sun, P. Peng, M. S. Dargusch,

and X. Yao, Philos. Mag. Letts. 90 (2010) 225.

[6] ] J. Zhou, Z. Zou, A.K. Ray and X.S. Zhao, Ind. Eng. Chem. Res. 46 (2007) 745.

[7] D. Gout, E. Benbow, O. Gourdon and G.J. Miller, J. Sol. Stat. Chem, 176 (2003) 538.

[8] A. Abragam and B. Bleaney, “Electron Paramagnetc Resonance of Transition Ions”

(

,


109

[9] H.A. Harwig, Z. Anorg. Allg. Chem. 444 (1978) 151.

[10] Y. Li, X. Wei, M. Yin and Y. Tao, Opt. Mater. 33 (2011) 1239.

CHAPTER-VI

Structure-property relations in New Bi2+xYbxPd4-3xO7+δ

6.1 Introduction

(x = 0.0, 0.25, 0.50 and 0.75)

Ni based oxides is characterized by very strong electron correlation effects. Such

effects in Pd based oxides have not been deeply studied. In contrast to the cubic

antiferromagnetism NiO, the nonmagnetic oxide PdO crystallizes in a tetragonal

structure. Ytterbium containing oxides are promising for many applications1. A

phenomenological model in which the 4f electrons of rare-earths polarize their 5d bands

and there are 5d-3d short range interactions with transition metals was proposed. The

local 4f-5d exchange interactions are positive and thus the 5d band polarizations are

parallely oriented to 4f moments. These unusual properties arise mainly due to the

110

hybridization between the moment carrying 4f and 5f electrons and the conduction

electrons.

The electronic structure of palladium has probably been the most thoroughly

studied of all the 4d transition metals. Its band structure has been reported by many

authors2. The configurations of the upper valence bands and lower conduction bands in

both oxides are strongly influenced by the d states of the cations. It is, therefore, of

considerable interest to study the physical properties. To develop a new material with a

desired band structure, it is essential to understand its electronic structure, particularly

near the Fermi level. Fundamental understanding of gapless band structure of that can be

a basis to design new material systems with novel physical properties, by tuning its band

structure, in the fields of magnetism,3 optics,4 electronics,5 and spintroics6

.

6.2 Results and discussions

6.2.1 Powder X-ray diffraction Studies

The powder X-ray diffraction patterns of Bi2+xYbxPd4-3xO7+δ (x = 0.0, 0.25, 0.50

and 0.75) (C1-C4) are shown in Fig.6.1. The powder X-ray diffraction patterns of the

samples are similar, and the major diffraction peaks are appeared in the same diffraction

angle shows that samples (C1-C4) are formed in a single phase polycrystalline materials

and the incorporation of Bi3+ and Yb3+ ions does not show any phase changes in the samples

within the atomic concentration range (0.0 ≥ x ≥ 0.75) studied. The unitcell in the sample is

found out to be tetragonal unitcell with P4/NCC. The cell parameters are shown in Table

6.1 along with the observed densities of the samples. Observed density values done using

liquid displacement method are compared with the calculated ones and the Z value is

111

found out to be 1. The cell dimension increases for C4 enormously and the space group

also changed to P4BM. This is attributed to large x values and higher valent Bi3+ and

Yb3+

6.2.2 SEM and EDX Micrographs

in the lattice.

SEM images of the samples C1-C4 were shown in Fig.6.2.1 (a-d). The surfaces

of the four samples show decrease in particle size with increasing Bi and Yb content. It

is shown that the particle morphology of high resolution the particle are most irregular in

shape with a nanosize range. This is also confirmed with crystallite sizes calculated

using XRD results by Scherrer relation. The overall porosity of the samples seems to be

almost identical. EDX profiles (Fig. 6.2.2 (a-d)) show good agreement with the

stoichiometry of the expected ratio of the samples.


sizes in C1-C4 of Bi2+xYbxPd4-3xO7+δ

Unit cell data/Sample C1 C2 C3 C4


________________________________________________________________________

________________________________________________________________________ Cell Formula Bi2Pd4O7 Bi2.25Yb0.25Pd3.25O7 Bi2.5Ce0.5Ni2.5O7

Bi2.75Ce0.75Ni1.75O7

Lattice

parameter* a (Å) 8.614 8.615 8.615 10.904

c (Å) 5.911 5.910 5.906 7.113

Cell volume (Å3

Space group P4/NCC P4/NCC P4/NCC P4BM

) 438.63 438.58 438.37 845.82

Z 1 1 1 1

Calculated

112

density (g/cc) 3.6200 3.6787 3.7400 4.2962

Experimental

density (g/cc) 2.9775 3.4306 2.7685 4.2652

Average crystallite

size (nm) 45 36 35 28

________________________________________________________________________

* before refinement

6.2.3 Thermal analysis

The DTA/DSC/TGA traces in the range 30-1000 °C (Fig. 6.3) shows that

the samples are stable in temperature up to 700 °C. DTA curves show an endothermic

peak with weight loss around 630 ºC. DTA/DSC traces show an endothermic peak at ~

600 ⁰C which corresponds to the reduction of Pd2+ to Pd+

ions in all the samples. An

endothermic peak around 950 °C may due to the phase change.

113

Fig. 6.1 powder X-ray diffraction pattern of Bi2+xYbxPd4-3xO7+δ

(0.25 ≤ x ≤ 1.0) C1-C4

114

(a) (b)

(c) (d)

Fig. 6.2.1. (a-d) SEM micrographs of Bi2+xYbxPd4-3xO7+δ

(0.25 ≤ x ≤ 1.0) C1-C4

115

(a)

(b)

Fig.6.2.2 (a-b) EDX profile of C1 and C2

116

(c)

(d)

Fig.6.2.2 (c-d) EDX profile of C3 and C4

117

Fig. 6.3 TGA/DTA/DSC traces of C1-C4

118

6.2.4 Crystal structure refinement and electron density Fourier mapping

The unit cell structure (Fig. 6.4.1 (a-d)) of the samples C1-C4 is developed on

space group P4//NCC with Z=1. Fig.6.4.2 (a-d) is shown the 2-dimensional view on

(001) plane. The Bi3+ ions are put in 4(c) sites; Pd2+(or Al3+) ions in 4(a); O2- ions in 8(f)

sites. Table 6.4.1, 6.4.2, 6.4.3 (a-d) also show the refined atomic position coordinates,

bond lengths and bond angles in the asymmetric units of C1-C4, respectively.

Asymmetric units (Fig. 6.4.3 (a-d)) show no changes in the structure. At first sight, it

appears that, no significant changes are observed in the bond lengths suggesting that the

total bonding interaction is practically constant in the composition range studied. The

ratio of the lattice parameters c/a is 0.6862, 0.6860, 0.6855 and 0.6523 for C1-C4

respectively. As the Yb concentration increases, the c/a ratio decreases in from 0.6862 to

0.6523 which means there is a slight increase in the tetragonality of the samples.

Selected bond lengths are shown in table. Pd-O interatomic distance is shorter than the

bond lengths usually observed7

The Fourier mapping of the 3-dimensional electron density from xz-plane (010),

and the 2-dimensional electron density contour on (010) plane are shown in Figs. 6.4.4.

(a-d) and 6.4.5(a-d) respectively.

. The average Pd-O distances vary from 1.96 to 1.84 Å.

Table 1 also shows that the average M-O-M (M = Pd or Bi) bond angle in the distorted

angle is 78.6 – 79.2° in C1-C4 respectively, compared to 109.5° for a regular tetrahedron.

119

(a) (b)

(c) (d)

Fig. 6.4.1 (a-d) Perspective view of unitcell structure of Bi2+xYbxPd4-3xO7+δ

1.0) C1-C4

(0.25 ≤ x ≤

120

(a) (b)

(c) (d)

Fig. 6.4.2. (a-d) 2-dimensional view on (001) plane of Bi2+xYbxPd4-3xO7+δ

C1-C4

(0.25 ≤ x ≤ 1.0)

121

(a ) (b)

( c) (d)

Fig.6.4.3. (a-d) Asymmetric unit of Bi2+xYbxPd4-3xO7+δ

(0.25 ≤ x ≤ 1.0) C1-C4

122

Table 6.4.1 Generated positions of atoms in the asymmetric unit Bi2+xYbxPd4-3xO7+δ

________________________________________________________________________

(0.25 ≤ x ≤ 1.0) C1-C4

Before Refinement After Refinement

No name crystal coordinates cartesian coordinates cartesian coordinates

x y z x y z x y z

________________________________________________________________________ C1

1 Pd 0.0000 0.0000 0.2500 -4.3072 -4.3072 -1.4777 -4.3092 -4.3092 -1.4775

2 Bi 0.0000 0.5000 0.0000 -4.3072 0.0000 -2.9554 -4.3092 0.0000 -2.9550

3 O1 0.3384 0.3384 0.2500 -1.3921 -1.3921 -1.4777 -1.3927 -1.3927 -1.4775

C2

1 Pd 0.0000 0.0000 0.2500 -4.3072 -4.3072 -1.4777 -4.3093 -4.3093 -1.4770

2 Bi 0.0000 0.5000 0.0000 4.3072 0.0000 -2.9554 -4.3093 0.0000 -2.9540

3 O1 0.3384 0.3384 0.2500 -1.3921 -1.3921 -1.4777 -1.3928 -1.3928 -1.4770

C3

1 Pd 0.0000 0.0000 0.2500 -4.3079 -4.3079 -1.4764 -4.3106 -4.3106 -1.4753

2 Bi 0.0000 0.5000 0.0000 -4.3079 0.0000 -2.9527 -4.3106 0.0000 -2.9506

3 O1 0.3384 0.3384 0.2500 -1.3923 -1.3923 -1.4764 -1.3932 -1.3932 -1.4753

C4

1 Pd 0.0000 0.0000 0.2500 -4.0287 -4.0287 -1.4932 -4.0340 -4.0340 -1.4927

2 Bi 0.0000 0.5000 0.0000 -4.0287 0.0000 -2.9864 -4.0340 0.0000 -2.9853

3 O1 0.3384 0.3384 0.2500 1.3021 1.3020 -1.4932 -1.3038 -1.3038 -1.4927

________________________________________________________________________

123

Table 6.4.2 Bond angles between the atoms _________________________________________ No atom1 atom2 atom3 angle quant _________________________________________ C1 Pd O1 Bi 78.6778 1

C2 Pd O1 Bi 78.6707 1

C3 Pd O1 Bi 78.6598 1

C4 Pd O1 Bi 79.2440 1

_________________________________________ Table 6.4.3 (a) Bond length of C1 _______________________________________ S. No. Atom1 Atom2 Bond length quant _______________________________________ 1 Bi O1 3.5536 32

2 O1 Pd 1.9696 8

_______________________________________ Table 6.4.3 (b) Bond length of C2 _________________________________ S. No atom1 atom2 distance quant ________________________________ 1 Bi O1 3.5535 24 2 O1 Pd 1.9697 8 ________________________________ Table 6.4.3 (c) Bond length of C3 ______________________________________ S. No atom1 atom2 distance quant ______________________________________ 1 Bi O1 3.5537 32 2 O1 Pd 1.9702 8 ____________________________________ Table 6.4.3(d) Bond length of C4 _________________________________ No atom1 atom2 distance quant _________________________________ 1 Bi O1 3.3737 32 2 O1 Pd 1.8438 8 _________________________________

124

(a) (b)

(c) (d)

Fig. 6.4.4 (a-d) 3-dimensional electron density from (010) plane of Bi2+xYbxPd4-3xO7+δ

(0.25 ≤ x ≤ 1.0) C1-C4

125

(a) (b)

(c ) (d)

Fig. 6.4.5 2-dimensional electron density contour on (010) plane of Bi2+xYbxPd4-3xO7+δ

(0.25 ≤ x ≤ 1.0) C1-C4 showing the symmetric positions of Bi, Pd and O atoms

on the plane.

126

6.2.5 Band structure and density of states

Materials can be broadly classified into metals, insulators, semiconductors, and

half metals, depending on their electronic band structures. If the conduction and valence

band edges meet at the Fermi level, the material belongs to a new class of solids named

zero-gap materials or gapless semiconductors.

The Figs. 6.5.1 (a, c, e, g) and Figs. 6.5.1 (b, d, f, h) show the electronic energy

band structures and the density of states, of The valence band VB is dominated by a low

energy region with s states of ytterbium than the bismuth ones due to the

electronegativity difference between these two (Bi – 2.02 and Yb – 1.1) at lower energy.

Their p states extend over the higher part of the VB, where they mix with the Pd 4d states

which are broadened over ~ 6 eV. This is due to their belonging to the second transition

metal series and to their mixing with the Bi p states as detailed in the narrow energy

PDOS shown in Fig. 6.5.2. As palladium is a late element of the 4d series, the large

filling of its d states upto 8.4 electrons makes them centered well below the Fermi level

(EF). Then, EF is crossed by low intensity itinerant like s, p like states arising from all

constituents. In a band structure description, they will be signaled by a large dispersion

band, i.e. contrary to flat localized bands characterizing d (and f) subshells. The band

gap opening in Pd containing oxide is a consequence of the tetragonal crystal field

splitting of the Pd 4d orbitals.8 Theoretically, within the local density approximation

(LDA) for the exchange and correlation functional9, 10 the reported band gap is 0.1 eV, or

even no gap at all8. The GGA calculations performed in the present work were also

unable to reproduce the gap. Band gap underestimation in semiconductors is a well

known limitation of local functional but semiconducting properties of Palladium oxide

surfaces remains an interesting open question. A large density of states is present at EF

127

for the Pd atom if the spin parity constraint is imposed, which might induce weak

localized ferromagnetic behavior11

The valence bands lying about between -17 and -15 eV, and between about -4 and

2 eV are almost contributions from oxygen 2p states. The DOS is higher for O 2p states

(the largest DOS of 25 electrons/eV) than Bi 6p states (the largest DOS of 8

electrons/eV). It means that some of the electrons from Bi 6p transform into the valence

bands and take part in the covalence interactions between Bi and O atoms. It is shown

that peaks around -17 and -5 to2 eV completely attribute to Pd 4d and 5p states. The band

gap energies of --eV, which are overly underestimated values as compared to the

experimental band gap energies eV.

.

6.2.6 Electron Paramagnetic Resonance Spectra

The EPR linshapes of the samples recorded at 300 K and 77 K are shown in Fig.

6.6 (a-b). The Pd2+ ion has a 4d8 electronic configuration, so that paramagnetic centers

are produced upon capture of either an electron or a hole. Although one of its isotopes

has a nuclear spin of 5/2, (105Pd, 22% natural abundance)12, those obtained in the present

work show hyperfine splitting due to interaction with this nuclear moment but not

reported earlier in oxide system. All 77.8 % of the Pd isotopes have (a zero nuclear spin)

a hyper fine splitting line sextet caused by the interaction of the unpaired electron with

the l05Pd. The observed EMR lineshapes at 300 K show g-values (Table 6.6) due to Pd+

(4d9) (105Pd, I=5/2, abundance ~ 22.2%) ions in axially distorted octahedral coordination.

At 77 K the observed g-values are g1 = 3.509, 3.434, 3.437, 3.442 due to Yb3+ ions

(171Yb, 4f13, S=1/2, I=1/2)12 and g2 = 2.388, 2.409, 2.411, 2.415 due to Pd+

ions in C1-C4,

respectively.

128

(a) (b)

(c) (d)

Fig. 6.5.1 (a) band structure (b) density of states of C1; (c) band structure (d) density of states of C2

129

(e) (f)

(g) (h)

Fig. 6.5.1 (e) band structure (f) density of states of C3; (g) band structure (h) density of states C4

130

Fig.6.5.2 Partial density of states of individual atoms of Bi, Pd, Yb and O of C1 and C4

131

Table 6.6 Observed g – values in Bi2+xYbxPd4-3xO7+δ

________________________________________

(0.25 ≤ x ≤ 1.0) C1-C4

S. No. giso at300 K giso ______________ ____________

at 77 K

g1 g2 g1 g2 ________________________________________

C1 2.833 2.085 3.509 2.388

C2 2.833 2.053 3.434 2.409

C3 2.874 2.086 3.437 2.411

C4 2.833 2.108 3.442 2.415

________________________________________



), total area of

2+xYbxPd4-3xO7+δ

0.75) system.

(0.0 ≤ x ≤

___________________________________________________________________

S.No mass magnetization coercivity retentivity, Mr magnetic (mg) Ms (μemu) Hci, G (μemu/G) susceptibility X 10-3

___________________________________________________________________ (emu/g)

C1 40.0 175.080 8154.60 80.751 5.469

C2 45.0 479.950 351.30 30.827 7.240

C3 47.0 2.690 442.01 108.450 0.565

C4 47.0 1.772 361.90 53.928 0.031

___________________________________________________________________

Table 6.8. Direct band gap energy values of C1-C4

________________________________________ Sample code band gap energy (eV) ________________________________________ C1 1.06 C2 1.06 C3 1.09 C4 1.11 ________________________________________

132

Fig. 6.6 (a) EPR spectra at 300 K of Bi2+xYbxPd4-3xO7+δ

(0.25 ≤ x ≤ 1.0) C1-C4

133

Fig. 6.6 (b) EPR spectra at 77 K of Bi2+xYbxPd4-3xO7+δ

(0.25 ≤ x ≤ 1.0) C1-C4

134

6.2.7 Magnetic behavior at RT by VSM

Room temperature ferromagnetic behavior of the samples C1-C4 can be clearly

observed from the hysteresis loops shown in Fig. 6.7.1. The coercive field (Hc)

(Table6.7) of the samples C1-C4 decreases with increasing the concentration of Yb

within the composition range studied. In addition, it is observed that the magnetization

also decreases with increasing the concentration of Yb3+ in the samples C1-C4. The area

under the hysteresis loop is decreasing from C1 to C4. This may be due to the existence

of superparamagnetic and paramagnetic atoms in the particles. They may have potential

applications in various fields, such as high-density information storage and spintronic

devices13

6.2.8 Optical absorption studies

.

Fig. 6.8 (a) shows the observed optical absorption spectra. As can be seen from

the spectra, there does not appear to be a clear dependency of the energy gap on the rare

earth present in the structure. Absorption band between 400 and 500 nm assigned to Pd2+

ions and attributed to d–d transitions14. Two weak absorption bands associated to Pd2+

charge transfer and Pd-O were observed at 224 and 283 nm15. The brownish color of the

samples is believed to be due to the presence of Pd2+ species bound to the oxygen of the

support16. The band gap energy were calculated by plotting (αhν)2 vs. hν shown in Fig.

6.8 (b). The band gap energy value increases from C1 to C4 marginally shown in Table

6.8. However the band gap values calculated from band structure and density of states

are varied from the band gap energy values calculated by optical absorption studies and

this underestimation is mainly due to the well known limitation of exchange correlation

functions.

135

Fig. 6.7.1 Magnetic hysteresis loops of Bi2+xYbxPd4-3xO7+δ

at 300 K

(0.25 ≤ x ≤ 1.0) C1-C4

136

Fig.6.8 (a) Observed optical absorption spectra of Bi2+xYbxPd4-3xO7+δ

1.0)

(C1-C4: 0.25 ≤ x ≤

Fig. 6.8 (b) Band gap energy calculation by plotting (αhν)2

vs. hν

1 2 30

1

2

(αhν

)2 X

1010

(cm

-1 e

V 2 )

hν (eV)

C1 C2 C3 C4

137

6.3 Conclusion

The Chapter concludes that the results and discussion of the Bi2+xYbxPd4-3xO7 (x

= 0.0, 0.25, 0.50 and 0.75) (System-C) (C1-C4) nanocomposites. Analysis of the powder

XRD patterns by Fullprof shows tetragonal unit cell with lattice parameters in C1-C4: a =

8.614, 8.615, 8.615, 10.904 Å; c = 5.911, 5.910, 5.906, 7.113 Å and space group

P4/NCC. The observed density values of C1-C4 are 2.9775, 3.4306, 3.7685, 4.2652 g/cc,

respectively. These values are compared with the calculated ones (C1-C4: 3.6200,

3.6787, 3.7400, 4.2962 g/cm3) and the Z value is found out to be 1. The DTA/DSC/TGA

traces in the range 30- 1000 °C shows that the samples are stable in temperature up to

940 °C. SEM micrographs show similar globular nature for all samples but the size of

the crystallites decreases when the x-value (concentration of Pb3+ ions) increases from

0.0 to 0.75. Rietveld refinement of the unit cell structure developed on space group

P4/NCC, Z=1 with Wycoff sites: Bi3+ in 4(c), Pd2+ in 4(a), O2- in 8(f) in the samples

C1-C4 is shows the agreement factors: Rp= 94.14 %, Rwp= 94.96 % and Rexp=0.17 % for

the representative sample C1. The electronic energy band structure and density of states

in the samples are calculated DFT by using CASTEP code. The band gaps in the samples

~ 0.007-0.008 eV in C1-C4 show fairly metallic nature in the samples. The band in the

range ~ -5.0-1.8 eV, is attributed to the overlap of Pd-4d and Bi 6p bands with O-2p

bands. This also indicates well-defined Pd-O, Bi-O and Pd-O-Bi co-ordinations in all the

samples, and this result is in conformity with the bond lengths and bond angles calculated

using powder XRD results described earlier. The observed EMR lineshapes at 300 K

show g-values: g║ = 2.833, 2.833, 2.874, 2.833; g⊥ = 2.085, 2.053, 2.086, 2.108 due to

Pd+ (4d9) (105Pd, I=5/2, abundance ~ 22.2%) ions in axially distorted octahedral

138

coordination. At 77 K the observed g-values are g1 = 3.509, 3.434, 3.437, 3.442 due to

Yb3+ ions (171Yb, 4f13, S=1/2, I=1/2) and g2 = 2.388, 2.409, 2.411, 2.415 due to Pd2+ ions

in distorted octahedral coordination in C1-C4, respectively. The presence of indistinct

hysteresis loops in the magnetic moment versus magnetic field plots indicates possible

weak ferromagnetic nature in the samples. Since the shape of a hysteresis loop is

determined partly by the domain state such behavior is attributed to the widely distributed

domain sizes in the samples. The optical absorption spectra show band in the range ~

224-283 nm due to the O2- → Pd2+ ( 4d8) charge transfer and the wide band in the range

400-500 nm is attributed to Pd2+

d-d transitions in distorted octahedral coordination. The

band gap energy levels are calculated using observed optical absorption studies and

compared with the theoretical band gap by DFT calculations.

139

References

[1] P. Wachter and H. Boppart, “Proceedings of the International Conference on Valence

Instabilties, (Eds.) Z ¨ urich, (Amsterdam, North-Holland, 1982).

[2] S.W. Chen, S.C. Huang, G.Y. Guo, J.M. Lee, S. Chiang, W.C. Chen,Y.C. Liang,

K.T. Lu, and J.M. Chen, Appld. Phys. Lett. 99 (2011) 012103.

[3] (a) B. Sampedro, P. Crespo, A. Hernando, R. Litra´n, J.C. Sánchez-López, P. Crespo,

C. López-Cartes, A. Fernandez, J. Ramı´rez, J.G. Calbet, and M. Vallet, Phys. Rev.

Lett. 91 (2003) 237203.

(b) M. K. Bruska, I. Czekaj, B. Delley, J. Mantzarasa and A. Wokauna, Phys. Chem.

Chem. Phys., 13 (2011) 15947.

[4] Y. Chang, J. Xu, Y. Zhang, S. Ma, L. Xin, L. Zhu, and C. Xu, J. Phys. Chem. C 113

(2009) 18761.

[5] J. Wang and C. Liu, J. Phys. Chem. B 108 (2004) 1653.

[6] A. Twardowski, H.J.M. Swagten, and W.J.M. de Jonge, Phys. Rev. B 42 (1990)

2455.

[7] Y. Laligant, A. Le Bail and G. Ferey, J. Sol. Stat. Chem. 81 (1989) 58.

[8] K-T Park, D.L. Novikov, V.A. Gubanov and A. Freeman, J. Phys. ReV. B 49 (1994)

4425.

[9] K.C. Hass and A. Carlsson, E. Phys. ReV. B 46 (1992) 4246.

[10] R. Ahuja, S. Auluck, B. Jonhansson and M.A. Khan. Phys. ReV. B 50 (1994) 2128.

[11] R. Litrán, B. Sampedro, T. C. Rojas, M. Multigner, J. C. Sánchez-López, P. Crespo,

C. López-Cartes, M. A. García, A. Hernando, and A. Fernández, Phys Rev B 73

(2006) 054404.

[12] A. Abragam and B. Bleaney, “Electron Paramagnetc Resonance of Transition Ions”,

140

(Oxford, England: Oxford University Press, 1970)

[13] T. Tahmasebi, S.N. Piramanayagam, R. Sbiaa, H.K. Tan, R. Law, S. Lua, and T.C.

Chong, Phys StatSol 5 (2011) 426.

[14] A.S. Sass, V.A. Shvets, G.A. Savelyeva and V.B. Kazanskii, Kinet. Catal. 26 (1985)

1149.

[15] A.N. Pestryakov, V.V. Lunin, S. Fuentes, N. Bogdanchikova and A. Barrera, Chem.

Phys. Lett. 367 (2003) 102.

[16] V. Panin, N. R. Khasanova, C. Bougerol, W. Schnelle, G. Van Tendeloo, and E. V.

Antipov, Inorg. Chem. 49 (2010) 1295.

141

Chapter – VII

Structure-Property Relations in New Bi2+xGaxPd4-3xO7+δ

7.1 Introduction

(x = 0.15, 0.30, 0.45 and

0.60) Series

Transition metal oxides especially 4d metals are studied intensively since they

exhibit novel magnetic, electronic and optical properties. In particular, the study of

magnetism in clusters of palladium and in general 4d elements has attracted special

attention as these elements are nonmagnetic in bulk1. Ferromagnetic palladium

compounds’ experiments carried out by Taniyama et al.2 have evidenced the appearance

of magnetic moment in Pd clusters. Due to the strong surface energy anisotropy, these

clusters of Pd exhibit a noncrystallographic icosahedral structure instead of the typical

bulk fcc symmetry, which has been identified as the origin of this ferromagnetism.3

The d-shells of the 4d transition metal ions in transition metal oxides are more

extended than their 3d counterparts. This extended nature of 4d electrons considerably

enhances the electron lattice interaction, which may be a source of structure related phase

transitions. These more extended 4d orbitals also tend to have a greater overlap with each

other leading to the decrease of the intra-ionic Coulomb interaction, relative to the 3d

orbitals. Since the 4d and 5d orbitals are more diffuse than the 3d orbitals in NiO, one

could expect, however, that the electron interactions will be comparatively weak. If this

is the case, this in turn suggests that band theory should be valid for these two oxides.

Historically, Al has been the most common dopant, with others including B, Ga, In, Ge,

Si and F. Ga, in particular, has several potential advantages over Al and Ga is less

reactive than Al.

142

This chapter contains the results and discussions of Bi2+xGaxPd4-3xO7+δ

7.2 Results and discussions

(D1-D4:

x=0.15, 0.30, 0.45 & 0.60) (System-D) and comparing the results with the previous

chapters.

7.2.1. Powder X-ray diffraction Studies

The powder XRD pattern of oxides Bi2+xGaxPd4-3xO7+δ

The ratio of the lattice parameters c/a is 0.6869, 0.6871, 0.6854 and 0.6857 for

D1-D4 respectively. From the c/a ratio, it is observed that there is no change in the

tetragonality of the samples. In conclusion the progressive substitution of Ga and Bi by

Pd remains the tetragonal structure.

(D1-D4: x=0.15, 0.30,

0.45 & 0.60) are shown in Fig. 7.1. All the indexed diffraction patterns were confirmed

that the overall samples are in single phase, and no impurity peaks were identified in the

sample. All of the oxides crystallized in the tetragonal system All patterns analyzed by

Fullprof and the results show tetragonal phase as same in the case of system C. with

lattice parameter a = 8.6157, 8.6267, 8.6228 and 8.6256 Å; c = 5.9183, 5.9274, 5.9104

and 5.9148 Å, with the space group I4/mcm and z = 1. The observed density values D1-

D4: 3.365, 3.423, 3.425, 2.892 g/cc done by liquid displacement method are compared

with the density values calculated by XRD results.

143

Fig. 7.1 Powder XRD patterns of Bi2+xGaxPd4-3xO7+δ

(D1-D4: x=0.15, 0.30, 0.45

& 0.60)

144


sizes in Bi2+xGaxPd4-3xO7+δ

0.60) composite oxides

________________________________________________________________________

(D1-D4: x=0.15, 0.30, 0.45 & 0.60) (0.15 ≤ x ≤

Unit cell data/Sample D1 D2 D3 D4 ________________________________________________________________________ Cell Formula Bi2.15Ga0.15Pd3.55O7 Bi2.3Ga0.3Pd3.1O7 Bi2.5Ga0.5Pd2.65O7

Bi2.6Ga0.6Pd2.2OLattice

7

parameter* a (Å) 8.6157 8.6267 8.6228 8.6256 c (Å) 5.9183 5.9274 5.9104 5.9148 Cell volume (Å3

Space group I4/mcm I4/mcm I4/mcm I4/mcm ) 439.32 441.12 439.45 440.06

Z 1 1 1 1 Calculated density (g/cc) 3.557 3.472 3.422 3.354 Experimental density (g/cc) 3.365 3.423 3.425 2.892 Average crystallite size (nm) 27 30 32 31 ________________________________________________________________________

* before refinement

7.2 SEM Micrographs

SEM micrographs shown in Fig.7.2 (a-d) the morphology of the samples D1-D4,

where in at x=0.60 rod-like shape is formed. Micrographs show as the concentration of

Ga increases the shape of the particles changes from globular shape to nanorod-like shape

from D1 to D4. SEM micrographs show fairly similar morphology of the samples D3-D4

with distinct rod-shaped particles in D4 (x=0.60). Investigation of various regions of the

samples also show similar results with respect to the particle sizes and shapes.

145

(a) (b)

(c) (d)

Fig.7.2 (a-d) SEM micrographs of Bi2+xGaxPd4-3xO7+δ

at different magnifications

(D1-D4: x=0.15, 0.30, 0.45 & 0.60)

146

Fig. 7.3. TGA/DTA/DSC traces of Bi2+xGaxPd4-3xO7+δ

(D1-D4: x=0.15, 0.30, 0.45, 0.60)

147

7.2.3 Thermal analysis

TGA/DTA/DSC profile (Fig. 7.3) shows weight loss up to 400 ⁰C which is due to

moisture absorbed by the sample. DTA results show an endothermic peak with the weight

loss at around 600 ⁰C which corresponds to reduction of Pd ions or removal of O2 from

the samples. FT-IR spectra show the band 550 cm-1 corresponds to Pd-O stretching

vibration4

7.2.4 Structure Refinement and Electron Density Fourier Mapping

in samples D1-D4.

Rietveld analysis was carried out on each of the four samples. The final Rietveld

plots of the samples D1-D4 is shown in Fig. 7.4.1 (a-d). Structure of the unit cells are

developed using the space group I4/mcm with Wycoff sites, D1-D4: Bi3+ - 4(c), Pd2+ - 4(b),

O2- - 8(c). The XRD patterns were refined by tetragonal symmetry using the space group

Fm3m. Generated positions of the asymmetric unit of D1-D4 were shown in Table 7.4.1

and significant refinement values, crystallographic data, and R-values are listed in Table

7.4.2. Rietveld refinement of the unit cell structure show appreciably lowered agreement

factors Rp = 94.88, 93.78, 95.55, 93.43 %; Rwp = 95.81, 95.88, 96.63, 94.45 % and Rexp =

0.22, 0.91, 0.20, 0.26 % in D1-D4 respectively. Calculated bond lengths, bond angles

show only marginal variations. Bond distances and bond angles obtained from the

refinement for all compositions are listed in Table 7.4.3 and 7.4.4. The projections of

the unit cell structures of D1-D4 on <001> plane is shown in Fig.7.4.2 (a-d) while that

of the asymmetric unit on the <001> plane are shown in Fig. 7.4.3 (a-d). The Fourier

mapping of the 3-dimensional electron density from xy-plane (001), and the 2-

dimensional electron density contour on (010) plane are shown in Figs. 7.4.4. (a-d) and

7.4.5(a-d) respectively

148

(a) (b)

(c) (d)

Fig. 7.4.1 Perspective view of the unitcell of Bi2+xGaxPd4-3xO7+δ

0.45 & 0.60)

(D1-D4: x=0.15, 0.30,

149

Table 7.4.1 Generated positions of atoms in the asymmetric unit of Bi2+xGaxPd4-3xO7+δ

(D1-D4: x=0.15, 0.30, 0.45 & 0.60)

________________________________________________________________________

Before Refinement After Refinement

No name crystal coordinates cartesian coordinates cartesian coordinates

x y z x y z x y z

________________________________________________________________________

D1

1 Pd 0.0000 0.5000 0.2500 -4.3079 0.0000 -1.4796 4.3177 0.0000 -1.4697

2 Bi 0.0000 0.0000 0.0000 -4.3080 -4.3079 -2.9591 -4.3178 -4.3177 -2.9393

3 O 0.2500 0.2500 0.2500 -2.1540 -2.1540 -1.4796 -2.1589 -2.1589 -1.4697

D2

1 Pd 0.0000 0.5000 0.2500 -4.3124 0.0000 -1.4821 -4.3210 0.0000 -1.4775

2 Bi 0.0000 0.0000 0.0000 -4.3124 -4.3124 -2.9641 -4.3211 -4.3210 -2.9550

3 O 0.2500 0.2500 0.2500 -2.1562 -2.1562 -1.4821 -2.1605 -2.1605 -1.4775

D3

1 Pd 0.0000 0.5000 0.2500 -4.3114 0.0000 -1.4776 -4.3121 0.0000 -1.4780

2 Bi 0.0000 0.0000 0.0000 -4.3115 -4.3114 -2.9552 -4.3121 -4.3121 -2.9561

3 O 0.2500 0.2500 0.2500 -2.1557 -2.1557 -1.4776 -2.1561 -2.1560 -1.4780

D4

1 Pd 0.0000 0.5000 0.2500 -4.3114 0.0000 -1.4776 -4.3121 0.0000 -1.4780

2 Bi 0.0000 0.0000 0.0000 -4.3115 -4.3114 -2.9552 -4.3121 -4.3121 -2.9561

3 O 0.2500 0.2500 0.2500 -2.1557 -2.1557 -1.4776 -2.1561 -2.1560 -1.4780

________________________________________________________________________

150

Table 7.4.2 Reliability factors of Bi2+xGaxPd4-3xO7+δ

_____________________________ (D1-D4: x=0.15, 0.30, 0.45 & 0.60)

Sample Rp Rwp Rexp No _____________________________ D1 0.9488 0.9581 0.0022 D2 0.9378 0.9555 0.0091 D3 0.9588 0.9663 0.0020 D4 0.9343 0.9445 0.0026 _____________________________

Table 7.4.3 Selected bond angles for D1-D4 ____________________________________ No atom1 atom2 atom3 angle quant ____________________________________ D1 Pd O Bi 90.0000 1 D2 Pd O Bi 90.0000 1 D3 Pd O Bi 90.0000 1 D4 Pd O Bi 90.0000 1 ____________________________________

Table 7.4.4 Selected bond lengths of D1-D4

_____________________________ No atom1 atom2 distance quant _____________________________ D1 1 Pd O 3.0531 1 2 Bi O 3.3884 1 D2 1 Pd O 3.0554 1 2 Bi O 3.3939 1 D3 1 Pd O 3.0491 1 2 Bi O 3.3885 1 D4 1 Pd O 3.0502 1 2 Bi O 3.3899 1 ______________________________

151

(a) (b)

(c) (d)

Fig. 7.4.2 2-dimensional view on (001) plane of Bi2+xGaxPd4-3xO7+δ

(D1-D4: x=0.15,

0.30, 0.45 & 0.60) respectively

152

(a) (b)

(c) (d)

Fig. 7.4.3 (a-d) Asymmetric unit of Bi2+xGaxPd4-3xO7+δ

0.60) respectively

(D1-D4: x=0.15, 0.30, 0.45 &

153

(a) (b)

(c) (d) Fig. 7.4.4 (a-d) 3-dimensional electron density from (001) plane of Bi2+xGaxPd4-3xO7+δ

(D1-D4: x=0.15, 0.30, 0.45 & 0.60)

154

(a) (b)

(c) (d)

Fig. 7.4.5 (a-d) 2-dimensional electron density contour on (010) plane of Bi2+xGaxPd4-3x

O7+δ (D1-D4: x=0.15, 0.30, 0.45 & 0.60) respectively

155

7.2.5 Band Structure and Density of States

Calculated band structure and density of states (Fig.7.5.1 (a-h)) are done using

CASTEP. The calculated band structure, density of states and optical properties were

done using the commercial version of the software package CASTEP code. This

program evaluates the total energy of periodically repeating geometries based on density-

functional theory and the pseudopotential approximation. In this case only the valence

electrons are represented explicitly in the calculations, the valence-core interaction being

described by nonlocal pseudopotentials. The Fermi level is at 0 eV. A smearing of 0.5

eV was used to generate the DOS plots. Band gap is 0.015 eV in D1-D4, respectively.

Band gap increases about 0.007 eV as compared with C1-C4, because of the

substitutional effect of Ga3+

Between -4 and 3 eV, the DOS are mostly from Pd-4d, Ga-4p, Bi-6p and O 2p

orbitals. The p states extend over the higher part of the VB, where they mix with the Pd

4d states which are broadened over ~ 6 eV. This is due to their belonging to the second

transition metal series and to their mixing with the Bi and Ga p states as detailed in the

narrow energy PDOS shown in Fig. 7.5.2. As in the case of system-c (C1-C4) samples,

here also the large dispersed band is centered well below the Fermi level (EF). Then, EF

is crossed by low intensity itinerant like s, p like states arising from all constituents. In a

band structure description, they will be signaled by a large dispersion band, i.e. contrary

to flat localized bands characterizing d (and f) subshells. The band gap opening in D1-

D4 is a consequence of the tetragonal crystal field splitting of the Pd 4d orbitals

ions in D1-D4.

5. The

GGA calculations performed in the present work were also unable to reproduce the gap.

Band gap underestimation in semiconductors is a well known limitation. The band gap

156

energies of 0.007 eV, which are overly underestimated values as compared to the

experimental band gap energies eV.

A large density of states is present at EF for the Pd atom if the spin parity

constraint is imposed, which might induce weak localized ferromagnetic behavior6

7.6 Electron Paramagnetic Resonance Spectroscopy

. The

valence bands lying about between -17 and -15 eV, and between about -4 and 2 eV are

almost contributions from oxygen 2p states. The DOS is higher for O 2p states (the

largest DOS of 25 electrons/eV) than Bi 6p states (the largest DOS of 8 electrons/eV). It

means that some of the electrons from Bi 6p transform into the valence bands and take

part in the covalence interactions between Bi and O atoms. It is shown that peaks around

-3 to 1 eV completely attribute to Pd 4d and 4s states.

Figs. 7.6.(1-3) show the EPR spectra recorded at 300 K and 77 K. The oxidation

states of palladium ions and cationic distribution in the oxides could be studied by EPR

spectroscopy. The non-magnetic Ga is substituted by Pd ions EPR line shapes recorded

show the giso- values g1= 4.260, 4.201, 4.184, 4.186; g2= 2.024, 2.018, 2.010, 2.033 at

300 K, g3 = 4.423, 4.186, 4.209, 4.204; g4 = 2.148, 2.148, 2.041, 2.051 at 77 K and

respectively. At 77 K, g4 signal is observable. This signal shows hyperfine splitting due

to l05Pd, a 22.6% abundant isotope7 having a nuclear spin of 5/2. The signal with any

hyperfine structure at g2 recorded at 77 K was also observed and became isotropic at

higher temperature (300 K). The g-value ~2.010-2.148 show the presence of both Pd+

and Pd2+

ions present in D1-D4, respectively. The g-values ~ 4.184 - 4.423 are evidence

of the presence of magnetic impurity ions in the samples.

157

(a) (b)

(c) (d) Fig. 7.5.1 (a) band structure (b) density of states of D1; (c) band structure (d) density of States of D2

158

(e) (f)

(g) (h) Fig. 7.5.1 (e) band structure (f) density of states of D3; (g) band structure (h) density of States of D4

159

Fig. 7.5.2 Partial Density of states of Bi, O, Ga, Pd in D4

160

Table 7.6 Observed g - values in D1-D4 _______________________________________ S. No. giso at300 K giso ______________ ____________

at 77 K

g1 g2 g1 g2_______________________________________

D1 4.260 2.024 4.423 2.148 D2 4.201 2.018 4.186 2.148 D3 4.184 2.010 4.209 2.041 D4 4.186 2.033 4.204 2.051 _______________________________________

7.7 Magnetic behavior at RT by VSM

All the samples show ferromagnetic behavior at room temperature (Fig.7.7). The

presence of hysteresis loop with very low values of the coercivity, retentivity, magnetic

susceptibility ~10-5 emu/gG shows fairly weak ferromagnetic8 nature of the samples.

According to recent theoretical calculations,9,10 however, a slight enhancement of 4d

electron localization induced by reduced dimensionality, local symmetry changes, and

lattice expansion would give rise to ferromagnetism in Palladium ions. Magnetization

and coercivity values decrease from x = 0.15 to x = 0.60, because the magnetic Pd2+ ions

are substituted by non-magnetic Ga3+

Table 7.7 Observed magnetization (M

ions.

s), coercivity (Hci), retentivity (Mr

the hysteresis, magnetic susceptibility, weiss constant and exchange integral ), total area of

of Bi2+xGaxPd4-3xO7+δ

_____________________________________________________________________ (0.15 ≤ x ≤ 0.60) system.

S.No mass magnetization coercivity retentivity, Mr magnetic (mg) Ms (μemu) Hci, G (μemu/G) susceptibility X 10-1

_____________________________________________________________________ (μemu/g)

D1 87.0 401.920 2057.40 225.990 4. 760 D2 83.0 111.220 561.17 24.112 1.190 D3 67.0 182.060 1714.10 12.244 2.936 D4 67.0 424.870 458.31 116.580 8.533 _____________________________________________________________________

161


162


Fig. 7.6.3 EPR spectra shows the zoom in the field around 3000 G at 77 K of D1

and D2 respectively

163

7.8 Optical Absorption Studies

DRS spectra of D1-D4 is shown in Fig 7.8 (a). Optical absorption band between

400-500 nm is assigned to Pd2+ ions and attributed to d–d transitions11. In addition, the

brownish color of the spheres is believed to be due to the presence of Pd2+ species bound

to the oxygen of the support12. Direct band gap energy values are obtained by plotting

(αhν)2

Table 7.8 Direct band gap energy values of D1-D4

vs. hν shown in Fig 7.8 (b) and the band gap values are given in Table 7.8. The

values obtained from experimental optical absorption studies are match up to the band

gap values by theoretical DFT calculations which underestimate the experimental one

due to the shortcoming of the DFT calculations.

_________________________________________ Sample code Band gap energy (eV) _________________________________________ D1 1.38 D2 1.43 D3 1.40 D4 1.52 _________________________________________

164

Fig. 7.7 Magnetic hysteresis loops of D1-D4 at 300 K

165

Fig. 7.8 (a) Observed optical absorption spectra of D1-D4

Fig. 7.8 (b) Band gap energy of D1-D4 obtained by plotting (αhν)2

vs. hν

1.0 1.5 2.0 2.5

0.5

1.0

1.5

2.0

(αhν

)2 X

1010

(cm

-1 e

V 2 )

hν (eV)

D1 D2 D3 D4

166

Conclusions

This Chapter describes the results and discussions of Bi2+xGaxPd4-3xO7+δ (D1-D4:

x=0.15, 0.30, 0.45, 0.60) (System-D) where the diamagnetic Ga3+(3d10) in is incorporated

in place of Yb3+(4f13) as in Chapter-6. Powder XRD results analyzed by Fullprof show

tetragonal phase with lattice parameter a = 8.6157, 8.6267, 8.6228 and 8.6256 Å; c =

5.9183, 5.9274, 5.9104 and 5.9148 Å, space group I4/mcm and Z = 1. The observed

density values are D1-D4: 3.365, 3.423, 3.425, 2.892 g/cc. TG-DTA traces show weight

loss up to 400 ⁰C due the loss of water as observed in the case of B1-B4. DTA/DSC

traces show an endothermic peak at ~ 600 ⁰C which corresponds to the reduction of Pd2+

to Pd+ ions in all the samples. SEM micrographs show fairly similar morphology of the

samples D1-D4 with distinct rod-shaped particles in D4(x=0.60). The structure of the unit

cells are developed using the space group I4/mcm and Z=1 with Wycoff sites: Bi3+ in 4(c),

Pd2+ in 4(b), and O2- in 8(c). Rietveld refinement of the unit cell structure show appreciably


94.45 % and Rexp = 0.22, 0.91, 0.20, 0.26 % in D1-D4, respectively. The calculated bond

lengths, bond angles show only marginal variations in the samples. FT-IR spectra show

the band 550 cm-1 which corresponds to Pd-O stretching in samples D1-D4. The

calculated band structure, density of states and optical properties in the samples are done

using CASTEP programme package. The band gap in the samples is found to be ~ 0.015

eV in D1-D4, respectively. The band gap increases by ~0.007 eV in D1-D4 as compared

with those in C1-C4, because of the substitutional effect of Ga3+ ions in D1-D4. This

result further shows that the samples D1-D4 are tend towards semiconductor-like with

respect to C1-C4. The observed EPR lineshapes at 300 K show two g-values: g1= 2.024,

167

2.018, 2.010, 2.033; g2= 4.260, 4.201, 4.184, 4.186 at 300 K and also two g-values: g3 =

2.148, 2.148, 2.041, 2.051; g4 = 4.423, 4.186, 4.209, 4.204 at 77 K in D1-D4,

respectively. The g-values ~2.0 is due to the Pd+(4d8) (105Pd, I=5/2, abundance ~ 22.2%)

and Pd2+(4d9) (105Pd, I=5/2, abundance ~ 22.2%) sites in D1-D4. While the g-values ~ 4.0

is attributed to the presence of magnetic impurity ions in the samples. The presence of

hysteresis loops with very low values of the coercivity, retentivity, magnetic

susceptibility ~10-5 emu/gG shows fairly weak ferromagnetic nature of the samples at

300 K. Optical absorption band between 400 - 500 nm is assigned to the d–d transitions

in Pd2+

ions. Comparison of band gap obtained by optical absorption studies with band

gap calculated by DFT calculation is done and the underestimation can be justified.

168

References

[1] V. Kumar and Y. Kawazoe, Eur. Phys. J. D 24, (2003) 81.

[2] T. Taniyama, E. Ohta, and T. Sato, Europhys. Lett. 38, (1997)195.

[3] R. Litrán,1, B. Sampedro, T. C. Rojas, M. Multigner, J. C. Sánchez-López, P. Crespo,

C. López-Cartes,M. A. García, A. Hernando, and A. Fernández, Phys. Rev. B 73

(2006) 054404.

[4] J. Fujita, A.E. Martell and K. Nakamoto, J. Chem. Phys. 36 (1962) 324.

[5] K.-T. Park, D. L. Novikov, V. A. Gubanov and A. Freeman, J. Phys.ReV. B 49

(1994) 4425.

[6] B. Sampedro, P. Crespo, A. Hernando, R. Litra´n, J. C. Sa´nchez Lo´pez, C. Lo´pez

Cartes, A. Fernandez, J. Ramı´rez, J. Gonza´lez Calbet, and M. Vallet, Phys Rev Lett

91(2003) 237203.

[7] A. Abragam and B. Bleaney, “Electron Paramagnetc Resonance of Transition Ions”

(

,


[8] Y. Oba, T. Shinohara, T. Oku, J. Suzuki, M. Ohnuma and T. Sato J. Phys. Soc.

Japan. 78 (2009) 044711/1.

[9] A. Delin, E. Tosatti, and R. Weht, Phys. Rev. Lett. 92 (2004) 057201.

[10] S. Blugel, Phys. ReV. Lett. 68 (1992) 851.

[11] A.N. Pestryakov, V.V. Lunin, S. Fuentes, N. Bogdanchikova and A. Barrera, Chem.

Phys. Lett. 367 (2003) 102.

[12] V. Panin, N.R. Khasanova, C. Bougerol, W. Schnelle, G. V. Tendeloo,

and E.V. Antipov, Inorg. Chem. 49 (2010) 1295.

169

Chapter – VIII

Summary, Conclusions and Future work

The thesis concludes with Chapter-8 with highlights of the overall findings in our

study, and with proposed prospects for further studies in such series. The new series of

Bi2+xAxD4-3xO7+δ

TGA/DTA/DSC traces show the similar hygroscopic behavior as well as the

thermal stability of the samples in the various systems. The SEM micrographs show the

similar morphology in the samples in a system but marginal variations with the different

systems. The microstructure of the four systems varies wherein system-A show tetrapod-

like structure, system-B, -D show rod-like shapes and system-C show globular nature.

The EDAX results show the stoichiometry close to nominal in the samples. Structure of

the unit cells are developed using the corresponding space group and Rietveld refinement

(A=Al, Ce, Yb & Ga; D=Ni & Pd) nanocomposite oxides have been

prepared by sol-gel method via nitrate-citrate route and their structure-property relations

have been studied by various solid state experimental and theoretical techniques. Powder

XRD results show single phase in all the systems. System-A show cubic unit cell with

the space group Pm3n and Z = 4; system-B shows cubic unit cell with space group Pa-3

and Z = 4; system-C shows tetragonal unit cell with space group P4/ncc and Z = 1; and

system-D shows tetragonal unit cell with space group P42/ncm and Z = 1. This result

also shows that the same compositional formula does not imply the same phase in all the

systems. The monophasic nature of each system depends on the concentration of the ‘x’

value. All the four systems are having different value of ‘x’ because of the stability of

the phase and the structure. The average crystallite sizes calculated using Scherrer’s

relation is in nanometer range for all the samples in the four systems studied.

170

has done on the structure. Fourier electron density mapping convinces the structural

features. The similar electron density mapping justifies the same space group of all the

samples obtained in respective systems. FT-IR spectral results show the presence of Ni-

O and Bi-O bonds in system-A, Ni-O, Bi-O and Ce-O bonds in system-B, Pd-O bond in

system-C and system-D in conformity with the powder XRD results.

The calculated electronic energy band structure, density of states and optical

properties were done using the commercial version of the software package CASTEP

code. The results show semiconductor-like behavior in A1 to metallic character in A2-

A4; semiconductor-like in B1-B4; fairly metallic nature in C1-C4; and semiconductor-

like in D1-D4. A remarkable character in the DOS is that there is a narrow band centered

below the EF which can be viewed as the contribution of Ni atoms which are localized in

octahedral lattice, it is possible that the localized electron configuration is one of the

factors of the observed weak ferromagnetic behavior in A1-A4 and B1-B4. A large

density of states is present at EF

The EPR results in the range 6-300 K in system-A, and at 77 k and at 300 K in

systems-B, -C and -D show the oxidation states of the magnetic ions and cationic

distribution in the samples.

for the Pd atom if the spin parity constraint is imposed,

which might induce weak localized ferromagnetic behavior in C1-C4 and D1-D4. These

results help us to infer that in such series synthesis of monophasic nanocomposite oxides

are quite possible for exploitation of their various physical properties as well as their

applications as semiconductors or composite metals.

The samples in all the systems exhibit hysteresis loops compatible with weak

ferromagnetic nature of the samples at 300 K. Comparison of the magnetization of A1-

171

A4 with the magnetization of B1-B4, increase in one order of magnitude obtained in B1-

B4 due to the substitution of magnetic rare-earth ion whereas non-magnetic aluminium

ion substitution is done in A1-A4.

The magnetization of C1-C4 decreases with increasing the concentration of Yb3+

in the samples C1-C4. This may be due to the existence of superparamagnetic and

paramagnetic atoms in the particles. Magnetization and coercivity values decrease from

D1 to D4 , because the magnetic Pd2+ ions are substituted by non-magnetic Ga3+

DRS spectra show the optical transitions correspond to 3d and 4d metal ions.

The band gap energy values obtained by experimental optical absorption studies are

higher from band gap values calculated from band structure and density of states by DFT

calculations using CASTEP and this underestimation is mainly due to the well known

limitation of exchange correlation functions. However, as a kind of effective

approximation its relative calculation value was quite exact, and it did not affect

theoretical analysis on electronic structure analysis.

ions.

The magnetic anisotropy and coercive fields are important properties of soft

magnetic materials which find applications in the magnetic recording and reading

technologies in modern devices. These modern devices exclusively need soft magnetic

materials as its basic magnetic component. Soft magnetic materials are extensively used

in inductors which form a basic requirement in high technology areas. The electrical

properties of the nanocomposite oxides are also have to be studied. It is believed that the

electrical properties of the series studied should be enhanced due to the addition of Bi

ions in the systems. Catalytic activity of metals such as Ni and Pd is noteworthy and also

the interaction with group III-A metals and rare-earth ions bring about the intriguing

172

interest to analyze the catalytic behavior of these materials and the above studied

Bi2+xAxD4-3xO7+δ

(A=Al, Ce, Yb & Ga; D=Ni & Pd) nanocomposite oxides series

adequately suit these demands and are considered to shape the future of advanced

technology.

173

List of publications Publications: 1. B.B Das, Jubin Jose and M Yogapriya, Sol-gel Synthesis and Structure-Property Relations Silver Nanoparticles-Dispersed in GeO2

Matrix. Journal of Chemistry and Chemical Engineering Volume 4, 2010, 58-62.

2. B.B Das, R. Srinivassan and M Yogapriya, Structure-Property Relations in xWO3-0.31P2O5-0.31B2O3-(0.38-x)Cs2

O (0.01 ≤ x ≤ 0.28) Glasses by IR, UV and EPR Spectroscopic Studies Journal of Chemistry and Chemical Engineering 5, 2011, 369-375.

3. B B Das and M Yogapriya, Sol-Gel Synthesis, Crystal Structure and Electronic Properties in Bi2+xAlxNi4-3xO7+δ

(0.0 ≤ x ≤ 0.75) Composite Oxides Intern. J. Pure Appl. Chem. 1, 2011, 30-41.

4. Sol-gel synthesis and structure-property relations in Nd4-xCs2(1+x)Fe5-xZnxO14+δ

(0≤x≤1.0) oxide systems. B.B Das, D. Mohanty, M Yogapriya and R Govindarao Journal of Chemistry and Chemical Engineering 5, 2011.(in press)

Conferences:

1.Sol-gel synthesis and crystal structures of Bi2+xAl2xNi4-3xO8+δ(x=0, 0.25, 0.50, 0.75), B B Das, M Yogapriya

, J Jose, E V Pratheesh, M Ganesh and M Sudhakar. In: presented in National Seminar on “Applied Physical, Theoretical Chemistry” (APTChem-2009) calicut University Malappuram, Kerala. (20.03.09-21.03.09)

2.Sol-gel synthesis and crystal structures of Bi2+xAl2xNi4-3xO8+δ(x=0, 0.25, 0.50, 0.75), B B Das, M Yogapriya, J Jose, E V Pratheesh, M Ganesh and M Sudhakar, I n: presented in 6th

National Symposium and Conference on Solid State Chemistry and Allied Areas.(ISCAS-2009), November 19-21, PP25

3.Sol-gel synthesis and structure-property relations in Ag nanoparticles-dispersed on carbon matrix. B B Das, J Jose and M Yogapriya. In: presented in 6th

National Symposium and Conference on Solid State Chemistry and Allied Areas.(ISCAS-2009), November 19-21, PP31

174

4. Sol-gel synthesis, Crystal Structure, Magnetic and Electronic properties of Bi2+xCexNi4-3xO7+δ Nanocomposites, B B Das and M Yogapriya

. In: presented orally in National Seminar on Current Trends in Chemistry. (CTric-2011), March 4-5, OP-6.

5. Sol-gel synthesis, Crystal Structure, Magnetic and Electronic properties of Bi2+xCexNi4-3xO7+δ Nanocomposites, B B Das and M Yogapriya

6. Sol-gel synthesis, Crystal Structure, Magnetic and Electronic properties of Bi

. In: presented (poster) in National Seminar on Current Trends in Chemistry. (CTric-2011), March 4-5.

2+xCexNi4-3xO7+δ Nanocomposites, B B Das and M Yogapriya. In: presented (Invited lecture) in 2nd

International conference on Nanomaterials. (ICN-2012), January 12-15.

SOL-GEL SYNTHESIS, CRYSTAL STRUCTURE, MAGNETIC, …

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