Low-temperature thermoelectric and magnetic characteristicsof Ca2.9Bi0.1Co42xFexO9+d (0 £ x £ 0.10)

Ankam Bhaskar • Z.-R. Lin • Chia-Jyi Liu

Received: 19 October 2013 / Accepted: 25 November 2013 / Published online: 4 December 2013

� Springer Science+Business Media New York 2013

Abstract The effect of Fe ion doping on the low-temper-

ature thermoelectric and magnetic properties of Ca2.9Bi0.1

Co4-xFexO9?d (x = 0.00, 0.025, 0.05 and 0.10) have been

investigated. The samples were prepared by conventional

solid-state synthesis. The X-ray diffraction patterns revealed

that all the samples are single phase. The electrical resistivity

results indicated that all the samples obey the variable range

hopping in the low temperature regime. The T* (transition

temperature from Fermi liquid metal to incoherent metal)

was increased and the slope of A value (Fermi-liquid trans-

port coefficient) was decreased with increasing Fe content.

The thermopower of all the samples was positive, indicating

that the predominant carriers are holes over the entire tem-

perature range. The electrical resistivity, thermopower and

total thermal conductivity were decreased with increasing Fe

content. Among the doped samples, Ca2.95Bi0.10Co3.90

Fe0.10O9?d had the highest dimensionless figure of merit of

0.056 at 300 K. Magnetic measurements indicated that all

the samples exhibit a low-spin state of cobalt ion. The

effective magnetic moments were decreased with increasing

Fe content.

1 Introduction

Thermoelectric (TE) materials have been the focus of

attention for protecting the environment by saving energy

resources and reducing the release of CO2 into the atmo-

sphere. The efficiency of thermoelectric devices is deter-

mined by materials dimensionless thermoelectric figure of

merit, ZT = S2T/qj, where S, q, T, and j are the Seebeck

coefficient, the electrical resistivity, the absolute tempera-

ture, and the thermal conductivity, respectively [1–3].

Recently, the misfit cobalt oxides (Ca3Co4O9?d) have been

investigated extensively as potential thermoelectric mate-

rial because it has large S, low q, and low j [4–10]. The

crystal structure of Ca3Co4O9?d system consists of two

subsystems, viz., the distorted NaCl-type [Ca2CoO3] sub-

lattice and the CdI2-type [CoO2] sublattice, alternatively

stacking along the c-axis [11, 12]. Polycrystalline bulk

Ca3Co4O9?d samples are still at a relatively low level for

industrial applications. Many attempts have been made to

optimize the thermoelectric performance of Ca3Co4O9?d

by either partially substituting cations or using appropriate

fabrication methods such as hot pressing [13] or spark

plasma sintering techniques [14]. Partial replacement of

cations in Ca3Co4O9?d has been carried out on either the

Ca site [15–21] or the Co sites [6, 9, 22–24]. According to

previous reports, the Ca-site substitution by Bi has been

improved the electrical conductivity in Ca3Co4O9?d system

[17, 25–29]. On the other hand, the highest thermopower

was obtained for the Co-site substituted by Fe in

Ca3Co4-xMxO9?d (M = Fe, Mn, and Cu; x = 0, 0.05, and

0.1) system [22, 30]. Therefore, it is interesting to inves-

tigate the effects of simultaneous substitution of Bi at the

Ca site and Fe at the Co site in Ca3Co4O9?d. In this paper,

we report the low-temperature (\300 K) thermoelectric

and magnetic properties of Ca2.9Bi0.1Co4-xFexO9?d

(x = 0.00, 0.025, 0.05 and 0.10) system.

2 Experimental procedures

Polycrystalline samples of Ca2.9Bi0.1Co4-xFexO9?d (x =

0.00, 0.025, 0.05 and 0.10) were synthesized by conventional

A. Bhaskar � Z.-R. Lin � C.-J. Liu (&)

Department of Physics, National Changhua University of

Education, Changhua 500, Taiwan

e-mail: liucj@cc.ncue.edu.tw

123

J Mater Sci: Mater Electron (2014) 25:778–784

DOI 10.1007/s10854-013-1645-9

solid-state reaction from CaCO3, Co3O4, Fe2O3 and Bi2O3

powders. The resulting powders were then pressed into

parallelepiped and sintered in air at 900 �C for 24 h. The

phase purity of resulting powders was examined by a Shi-

madzu XRD-6000 powder x-ray diffractometer equipped

with Fe Ka radiation. Electrical resistance measurements

were carried out using standard four-probe techniques.

Thermopower measurements were performed between 75

and 300 K using steady-state techniques with a temperature

gradient of 0.5–1 K across the sample. A type E differential

thermocouple was used to measure the temperature differ-

ence between hot and cold ends of sample [31], which was

measured using a Keithley 2000 multimeter. The thermo-

power of sample was obtained by subtracting the thermo-

power of Cu Seebeck probe. Carrier concentration and

mobility were determined by Hall measurements using the

van der Pauw method under an applied field of 0.55 T

(ECOPIA: HMS-3000) [32]. Thermal conductivity mea-

surements were carried out using transient plane source

techniques with very small temperature perturbations of

sample material using a Hot Disk thermal constants analyzer

[33]. The transient plane source technique makes use of a

thin sensor element in the shape of a double spiral. The Hot

Disk sensor acts both as a heat source for generating tem-

perature gradient in the sample and a resistance thermometer

for recording the time dependent temperature increase. The

encapsulated sensor was sandwiched between two pieces of

samples. During a preset time, 200 resistance recordings

were taken, and from these a relation between temperature

and time was established [32]. A commercial superconduc-

ting quantum interference device magnetometer (Quantum

Design) was used to characterize the magnetic properties of

the samples. The excess of oxygen content and valence state

of cobalt were determined using iodometric titration [34].

3 Results and discussion

Figure 1a shows the X-ray diffraction patterns of

Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025, 0.05, and 0.10)

samples. The XRD patterns show that all the samples are

single phase, and no secondary phase is detected. All the

diffraction peaks for all the samples can be indexed based

on the monoclinic misfit structure with the superspace

group X2/m(0b0)s0 [4, 8–10, 35]. The similarity between

the XRD patterns of undoped and doped samples suggest

that substituting ions of Fe do not change the crystalline

structure. The structure refinement using a Jana2006 Ri-

etveld program is carried out for all the samples [36].

Figure 1b shows the Rietveld fits to power XRD data for

the Ca2.9Bi0.1Co3.95Fe0.05O9?d using the superspace group

X2/m/(0b0)s0. The calculated and the difference profiles

(Rp = 6.42 %, Rwp = 9.48 %, and GOF = 1.47) are in

good agreement with previous reports [8, 37]. Lattice

parameters are given in Table 1. The lattice parameters do

not show monotonic trend, which may be due to the dual

doped (Bi and Fe ion at Ca-site and Co-site) and excess of

oxygen content. The radius of Bi3? is 1.03 A, which is

higher than that of Ca2? radius (1.00 A). The radius of low

spin (LS) Co4?, LS Co3?, intermediate spin (IS) Co3?, and

high spin (HS) Co3? is 0.53, 0.545, 0.56, and 0.61 A,

Fig. 1 a XRD patterns of Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025,

0.05 and 0.10) samples, b Observed (dotted type), calculated (solid

line), and difference powder XRD profiles for the Ca2.9Bi0.1Co3.95

Fe0.05O9?d using the superspace group X2/m/(0b0)s0

Table 1 Lattice parameters of the Ca2.9Bi0.1Co4-xFexO9?d

(x = 0.00, 0.025, 0.05 and 0.10) samples (b axis lengths b1:

Ca2CoO3, b2: CoO2 sublattice)

x a (nm) b1 (nm) b2 (nm) c (nm) b (�)

0.00 0.4830(1) 0.4562(9) 0.2817(3) 1.0846(9) 98.09(5)

0.025 0.4827(1) 0.4557(6) 0.2816(8) 1.0844(4) 98.10(1)

0.05 0.4828(4) 0.4551(1) 0.2817(6) 1.0850(3) 98.10(1)

0.10 0.4829(4) 0.4563(6) 0.2818(1) 1.0858(3) 98.10(1)

J Mater Sci: Mater Electron (2014) 25:778–784 779

123

respectively, while the radius of LS Fe3?, HS Fe3?, and

Fe4? is 0.55, 0.645, and 0.585 A, respectively. Karppinen

et al. [38] reported that lattice parameters (a and c) are

increased with increasing excess of oxygen content for the

Ca3Co3.95O9?d (d = 0.00, 0.24, 0.29) system. Iodometric

titration results show that the excess of oxygen content

increases with increasing Fe content (Table 2). Therefore,

one can conclude that lattice parameters are random due to

the dual-doped and excess of oxygen content. The oxygen

vacancies are located in the rock salt-type [Ca2CoO3]

subsystem [39–41]. The crystal structure of Ca3Co4O9?d

consists of two subsystems; these are triple rock salt-type

[Ca2CoO3] block (subsystem 1) and a CdI2-type hexagonal

[CoO2] block (subsystem 2). Therefore, the Fe ion can

occupy in either [Ca2CoO3] or [CoO2] block. The bulk

density was measured by applying Archimedes principle at

room temperature. The densities of samples are

*3.92–4.02 g/cm3, which are 79–81 % of theoretical

density (4.94 g/cm3). This value is in relatively good

agreement with earlier reports [4, 22, 30, 42–44]. Kenfaui

et al. [13, 42] conducted research on the Ca3Co4O9

ceramics prepared by solid-state reaction method and

ceramics were sintered by different sintering techniques

such as conventional sintering (CS), hot pressing (HP),

spark plasma sintering techniques (SPS). They have

obtained the bulk density of 2.82 g/cm3 (CS), 4.45 g/cm3

(HP), 4.59 g/cm3 (SPS), which corresponds to 60, 95 and

98 % of the theoretical density, respectively. Wang et al.

[22, 30], Pinistoontorn et al. [43, 44] also reported that the

bulk density of samples is in the range of &3.2–3.8 g/cm3

for the doped and undoped of Ca3Co4O9?d samples; this

value is 65–80 % of theoretical density.

Table 2 summarizes the characterization and properties

for the Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025, 0.05,

and 0.10) samples. The parent (Ca2.9Bi0.1Co4O9?d) sample

shows the highest resistivity among the samples. The

resistivity of undoped sample is 0.0120 X-cm at 300 K.

The magnitude of q for all the doped samples is in the

range of 0.0119–0.0107 X-cm at 300 K. The resistivity of

doped samples is higher than that of previous report [35].

Bhaskar et al. [35] have observed that electrical resistivity

is in the range of 0.0100–0.0093 X-cm at 300 K for the

Ca2.95Bi0.05Co4-xFexO9?d (x = 0.00, 0.05, 0.10 and 0.15).

This difference can probably be attributed to the higher

content of Bi at Ca-site in Ca3Co4O9?d system, which will

reduce the hole carrier concentration. The resistivity of

samples increases with decreasing Fe content due to a

increase in hole carrier concentration (Table 3). The

valence of Fe ion (Fe2? or Fe3?) is lower than that of the

average valence of Co (Table 2). On the basis of valence

equilibrium, the substitution of Fe ion for Co ion introduces

positive charge carriers into the system, which will enhance

the hole carrier concentration. Hall effect measurements

reveal that the hole concentration increases for the doped

samples as compared to the undoped sample (Table 3).

Similar results were also found in earlier reports [22, 30].

Wang et al. [22, 30] reported that the hole carrier con-

centration increases with increasing Fe doping level for the

Ca3Co4-xFexO9?d with x = 0.00–0.10. Besides, the excess

of oxygen content also creates the hole carriers into the

system. Karppinen et al. [38] reported that excess of oxy-

gen content decreases the resistivity of samples for the

Ca3Co3.95O9?d (d = 0.00, 0.24, 0.29). Iodometric titration

results show that the excess of oxygen content increases

with increasing Fe content (Table 2), indicating that the

hole carrier concentration increases and hence decreases

the resistivity. On the other hand, the average valence state

of cobalt increases with increasing Fe content; this may be

related to an increase in Co4?/Co3? ratio, leading to the

increase of hole carrier concentration (Co4?) [8, 37].

Therefore, these results suggest that the decrease of elec-

trical resistivity is mainly contributed by the Fe dopant and

excess of oxygen content.

Figure 2 shows the temperature dependence of resis-

tivity for the Ca2.9Bi0.10Co4-xFexO9?d (x = 0.00, 0.025,

0.05, and 0.10) samples. All the samples of electrical

resistivity decreases with increasing temperature, a typical

Table 2 Room temperature characterization of the Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025, 0.05 and 0.10) samples

x Cov? d q (X-cm) S (lV/K) jtotal (W/mK) jcarr (W/mK) jph (W/mK) ZT

0.00 3.127(4) 0.304(9) 0.0120 144 1.00 0.059 0.941 0.054

0.025 3.208(1) 0.463(5) 0.0119 139 0.90 0.060 0.840 0.054

0.05 3.218(1) 0.480(8) 0.0113 138 0.94 0.062 0.878 0.053

0.10 3.243(1) 0.523(9) 0.0107 130 0.85 0.066 0.784 0.056

Table 3 Carrier concentration and mobility of the Ca2.9Bi0.1

Co4-xFexO9?d (x = 0.00, 0.025, 0.05, and 0.10) samples

x Carrier

concentration

(1020 cm-3)

Mobility

(cm2/Vs)

0.00 2.38 2.16

0.025 2.39 2.17

0.05 2.41 2.26

0.10 2.50 2.30

780 J Mater Sci: Mater Electron (2014) 25:778–784

123

characteristic of nonmetallic-like (dq/dT \ 0) temperature

dependence, then increases with increasing temperature, a

typical characteristic of metallic-like (dq/dT [ 0) temper-

ature dependence. The metal to nonmetal transition (TMI) is

determined by dq/dT. The dq/dT is zero at transition

temperature. The TMI is around 88 ± 2 K for all the

samples. The metal to nonmetal transition occurs in the low

temperature region, which is similar to other elements

doped in Ca3Co4O9?d systems [4, 6, 8, 22, 23, 30, 35]. In

the insert of Fig. 2 shows the metal-like behavior in the

high temperature region from 120 to 300 K for the

Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025, 0.05 and 0.10)

samples.

In general, the resistivity behavior of a Fermi-liquid

system can be expressed by the following equation:

q ¼ q0 þ AT2 ð1Þ

where q0 is the residual resistivity owing to the domain

boundaries and other temperature-independent scattering

mechanisms, and A is the Fermi-liquid transport coefficient

[45–48]. Limelette et al. [45–48] reported that Ca3Co4O9

system exhibits two electrical resistivity characteristic

temperatures (TMI and T*) between 5 and 300 K, where

TMI is the metal to nonmetal transition temperature and T*

the transition temperature from Fermi liquid metal to

incoherent metal. The curves are fitted by Eq. (1) for

temperatures above TMI, and are shown in Fig. 3. The

transition temperature from Fermi liquid metal to inco-

herent metal (T*) is obtained at the end temperature of the

linear dependence in the metal-like range. The T* lies

between 217 ± 1 and 220 ± 1 K for the x = 0.00–0.10,

respectively. The slope of A value is obtained in the range

between 7.6 9 10-5 and 6.8 9 10-5 mX-cm K-2 for the

x = 0.00–0.10, respectively. According to the dynamical

mean field theory, a key role of effective mass m* of a

Fermi liquid is predicated as T* & 1/m*. The decrease of

A value and increase of T* indicates a decrease of m*,

which implies an increase in bandwidth and weakened the

electronic correlation in these system [49].

To facilitate the understanding of transport mechanism

of charge carriers in the nonmetallic regime, the natural

logarithm of reciprocal of resistivity is plotted against

T-1/4 in the nonmetallic regime. For variable-range hop-

ping transport in a disordered system, the temperature

dependence of resistivity would obey following relation at

low temperature [50].

r ¼ r0 exp � T0

T

� �1= dþ1ð Þ" #

ð2Þ

where r0 is weakly temperature dependent, T0 associated

with localization length, and d the dimensionality. For a

three-dimensional system, the conductivity ln r should

vary as T-1/4. Figure 4 shows the ln r versus T-1/4 for the

Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025, 0.05, and 0.10)

samples. This behavior might be associated with the

positional disorder positional disorder involved in the

incommensurate structure of the misfit layered title system

[35, 51].

Thermopower measurements are a very sensitive probe

to the type and characteristic energy of carriers and are a

complementary tool to the resistivity measurements for

transport studies. Since thermopower is a measure of the

heat per carrier over temperature, we can thus view it as a

measure of the entropy per carrier. The positive thermo-

power confirms that the dominant charge carriers are holes

Fig. 2 The temperature dependence of electrical resistivity for the

Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025, 0.05 and 0.10) samplesFig. 3 Variation of q versus T2 of Ca2.9Bi0.1Co4-xFexO9?d

(x = 0.00, 0.025, 0.05 and 0.10) samples. The solid lines are linear

fitting using q = q0 ? AT2. A: Fermi-liquid transport coefficient,

TIM: transition temperature of nonmetallic to metallic, T*: strongly

correlated Fermi-liquid regime up to the temperature

J Mater Sci: Mater Electron (2014) 25:778–784 781

123

for all the samples. Moreover, Hall measurements also

exhibit the majority carriers are p-type. The undoped

sample exhibits a larger absolute S value 147 lV K-1 at

300 K. The thermopower of undoped sample is higher than

that of the doped samples. The thermopower of doped

samples decrease with increasing Fe content due to an

increase in hole carrier concentration (Table 3). Besides,

the excess of oxygen content also affects the thermopower.

Karppinen et al. [38] reported that the thermopower

slightly decreases with increasing excess of oxygen content

for the Ca3Co3.95O9?d (d = 0.07, 0.24, 0.29) system. In our

case, the excess of oxygen content increases with

increasing Fe content (Table 2), indicates an increase in

hole carrier concentration and thus leads to a decrease of

thermopower. Therefore, one can easily conclude that

decreasing the thermopower is mainly contributed by

increasing the hole carrier concentration. In general,

increasing the carrier concentration of materials would

decrease the electrical resistivity and decrease the ther-

mopower. Figure 5 shows the temperature dependence of

thermopower (S) for the Ca2.9Bi0.1Co4-xFexO9?d

(x = 0.00, 0.025, 0.05 and 0.10) samples. The undoped

and doped samples exhibit a similar temperature depen-

dence of thermopower, but the absolute thermopower

values are different. Similar results were observed in other

elements doped in Ca3Co4O9?d system [6, 8].

In general, the thermoelectric power can be expressed

by the Mott formula [22, 52].

SðTÞ ¼ 1

eT

R1�1 rðeÞðe� lÞ of ðeÞ

oe deR1�1 rðeÞ of ðeÞ

oe deð3Þ

where r(e), f(e), and l represent electrical conductivity,

Fermi–Dirac distribution function at energy e, and

chemical potential. The product of the thermoelectric

coefficient and temperature can therefore be understood as

the mean energy flow carried by a conduction electron.

Using the condition of qf(e)/qe = d (e - EF), and

r = nel(e) [22, 52] Eq. (3) can be written as:

SðTÞ ¼ Ce

nþ p2k2

BT

3e

o ln lðeÞoe

� �e¼EF

ð4Þ

where l(e), Ce, and kB are the energy correlated carrier

mobility, electronic specific heat, and Boltzmann constant,

respectively. If S is inversely proportional to the n, it is

usually interpret as the predominance of first term in

Eq. (4). If the second term in Eq. (2) is dominant then S

closely related to the electronic structure [22, 30, 46].

Therefore, it can be concluded that first term is dominant in

our case.

Total thermal conductivity (jtotal) can be expressed as

jtotal = jcarr ? jph, where jcarr and jph represent the car-

rier and the lattice thermal conductivity, respectively. jcarr

can be calculated using the Wiedemann–Franz–Lorenz

relationship, jcarr = LrT, where L = p2k2/3e2 = 2.45 9

10-8 W X K-2 is the Lorenz number and T is the absolute

temperature. The total thermal conductivity slightly

decreases with increasing Fe content. For materials with

q[ 1 X-cm, jcarr is negligible. But in our case, the resis-

tivity is lower than 1 X-cm, a fact which leads us to

determine the jcarr by Wiedemann–Franz law. The calcu-

lated value of jcarr at 300 K is 0.059 and 0.066

W m-1 K-1 for the Ca2.9Bi0.1Co4O9?d and Ca2.9Bi0.1

Co3.90Fe0.10O9?d, respectively. For all the samples, the

lattice contribution is more important than the carrier one.

Due to the small jcarr, jtotal is mainly attributed to the

lattice contribution. The figure of merit (ZT = S2T/qj) is

calculated for all the samples. The highest ZT (0.056) is

reached for the Ca2.9Bi0.1Co3.90Fe0.10O9?d among the

Fig. 5 The temperature dependence of thermopower (S) for the

Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025, 0.05 and 0.10) samplesFig. 4 Plot of In r versus T-1/4 (K-1/4) for the Ca2.9Bi0.1Co4-x

FexO9?d (x = 0.00, 0.025, 0.05 and 0.10) samples

782 J Mater Sci: Mater Electron (2014) 25:778–784

123

samples, which is slightly improved as compared to the

undoped Ca2.9Bi0.1Co4O9?d at 300 K. The ZT value of our

samples is higher than those of previous reports [17, 22, 40,

53–55]. Wang et al. [22] have reported the highest ZT of

0.026 at 300 K for the Ca3Co4-xMxO9?d (M = Fe, Mn,

and Cu; x = 0, 0.05, and 0.1). Liu et al. [17] reported the

ZT value of 0.045 at 300 K for the Ca2.85Bi0.15Co4O9

samples sintered by SPS. Moser et al. [40] reported the ZT

of *0.04 at 300 K for the Ca2.8Bi0.2Co4O9?d samples

prepared by conventional solid-state reaction method. Park

et al. [53] reported a ZT value &0.03 at 300 K for the

Ca2.7Bi0.3Co4O9 sample prepared by conventional solid-

state synthesis. Li et al. [54] prepared Ca2.5Bi0.5Co4O9?d

sample by conventional solid-state reaction method and

reported a ZT value of *0.03 at 300 K. Xu et al. [55]

reported that the ZT value of *0.025 at 300 K for the

Ca2.7Bi0.3Co4O9?d sample prepared by hot-pressing tech-

nique. These results suggest there is scope for further

improvement of thermoelectric properties.

Figure 6 shows the temperature dependence of magnetic

susceptibility for the Ca2.9Bi0.1Co3.90Fe0.10O9?d sample.

The susceptibility value monotonically decreases with

increasing temperature. The observed effective magnetic

moment is derived by fitting the magnetic susceptibility

versus temperature using the Curie–Weiss law, as shown in

Fig. 6. The observed effective magnetic moments are

1.28 lB/Co for x = 0.00, 1.24 lB/Co for x = 0.05 and

1.15 lB/Co for x = 0.10, respectively. The observed

effective magnetic moments decrease with increasing Fe

content; this may be related to the increasing super

exchange mechanism, Fe3?–O2-–Fe3? or Fe2?–O2-–Fe2?.

Wang et al. [22] also reported that super exchange mech-

anism is enhanced for the Ca3Co3.9Fe0.1O9?d as compared

to undoped Ca3Co4O9?d sample. These results suggest that

ferrimagnetic is suppressed by the Fe substitution. The

interlayer coupling between CoO2 and Ca2CoO3 sublayers

could be the origin of ferrimagnetism [56–58]. The

Ca2CoO3 layer has two Ca–O planes and one Co–O plane,

where the Co–O plane is sandwiched by the two Ca–O

planes, and the Ca–O planes are located between the Co–O

plane and CaO2 sublayers [11]. The ferromagnetic cou-

pling would weaken by the distortion of Ca–O and Co–O

planes due to the bismuth substitution in Ca–O planes and

Fe substitution in Co–O planes. The Co3? and Co4? ions

with a low-spin configuration have a theoretical effective

magnetic moment of 0 lB/Co and 1.73 lB/Co, respec-

tively. The Co3? and Co4? ions with a high-spin configu-

ration have a theoretical effective magnetic moment of

4.98 and 5.91 lB/Co, respectively. The observed effective

magnetic moment is closely to the low-spin configuration

of cobalt. Liu et al. [7] reported that low-spin state of

cobalt is observed in Ca3Co4-xFexO9?d (x = 0.00, 0.05,

0.10, and 0.15) system.

4 Conclusions

In conclusion, we have prepared polycrystalline samples of

Ca2.9Bi0.1Co4-xFexO9?d (x = 0.00, 0.025, 0.05 and 0.10)

samples by conventional solid-state synthesis. The XRD

results show that all the samples are single phase. The

electrical resistivity results indicate that all the samples

obey the variable range hopping mechanism in the low

temperature regime. The TMI is observed at around

88 ± 2 K. The T* and slope of A value decrease with

increasing Fe content. The positive thermopower confirms

that the dominant carriers are holes for all the samples. The

substitution of Fe ion increases the hole carrier concen-

tration, which results in decrease of resistivity and ther-

mopower. The highest figure of merit (ZT) is observed for

the Ca2.9Bi0.1Co3.90Fe0.10O9?d among the samples. Mag-

netic measurements indicate that all the samples exhibit a

low-spin state of cobalt ion. The effective magnetic

moment decreases with increasing Fe content.

Acknowledgments This work was supported by National Science

Council of Republic of China, Taiwan under the Grant No. 101-2112-

M-018-003-MY3. Ankam Bhaskar would like to express thanks to the

postdoctoral fellowship sponsored by NSC of Taiwan.

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