X-ray diffraction, μ -Raman spectroscopic studies on CeO 2 − RE 2 O 3 ( RE = Ho , Er)systems: Observation of parasitic phasesB. P. Mandal, M. Roy, V. Grover, and A. K. Tyagi Citation: Journal of Applied Physics 103, 033506 (2008); doi: 10.1063/1.2837042 View online: http://dx.doi.org/10.1063/1.2837042 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/103/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Raman spectra of R 2O3 (R—rare earth) sesquioxides with C-type bixbyite crystal structure: A comparative study J. Appl. Phys. 116, 103508 (2014); 10.1063/1.4894775 High pressure phase transitions and compressibilities of Er 2 Zr 2 O 7 and Ho 2 Zr 2 O 7 Appl. Phys. Lett. 92, 011909 (2008); 10.1063/1.2830832 R -dependent magnetic and structural properties in R Mn 2 O 5 with R = Y , Er , Ho , Dy , and Tb J. Appl. Phys. 99, 08J508 (2006); 10.1063/1.2175866 The morphotropic phase boundary and dielectric properties of the x Pb ( Zr 1 ∕ 2 Ti 1 ∕ 2 ) O 3 - ( 1 − x ) Pb ( Ni 1 ∕3 Nb 2 ∕ 3 ) O 3 perovskite solid solution J. Appl. Phys. 96, 5103 (2004); 10.1063/1.1796511 Micro-Raman scattering and x-ray diffraction studies of ( Ta 2 O 5 ) 1−x ( TiO 2 ) x ceramics J. Appl. Phys. 87, 8688 (2000); 10.1063/1.373597

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X-ray diffraction, �-Raman spectroscopic studies on CeO2−RE2O3„RE=Ho, Er… systems: Observation of parasitic phases

B. P. Mandal, M. Roy, V. Grover, and A. K. Tyagia�

Chemistry Division, Bhabha Atomic Research Centre, Mumbai-400 085, India

�Received 31 August 2007; accepted 24 November 2007; published online 5 February 2008�

The phase relations in CeO2−Ho2O3 and CeO2−Er2O3 systems have been established under theslow-cooled conditions. As per x-ray diffraction �XRD�, in both the series a single-phasic solidsolution forms up to the nominal composition Ce0.6RE0.4O1.8 �RE=Ho, Er� retaining the F-typestructure of parent ceria. In Ce1−xErxO2−x/2 system the presence of microdomains of C-type phasehave been revealed by Raman spectroscopy for composition x=0.4, which has been identified assingle phasic by XRD. Photoluminescence studies also show that biphasic region commences fromx=0.4 for Ce1−xHoxO2−x/2 series. The biphasicity continues until x=0.7 for both the series. Fromx=0.8 the solid solutions exist as C-type single phasic, which is isotypic to another end memberRE2O3 �RE=Ho, Er� and as revealed by both XRD and Raman spectroscopy. High temperatureXRD studies show that no temperature induced phase change has been observed in either of theseries until 1273 K. In this work photoluminescence data was used to delineate the phase boundary.© 2008 American Institute of Physics. �DOI: 10.1063/1.2837042�

I. INTRODUCTION

Rare-earth-doped �RE-doped� ceria is an important ma-terial in view of applications in oxygen concentration cellsand in solid oxide fuel cells.1,2 It is also being considered asa potential catalyst for mitigating hydrocarbons, CO and NOx

from automobile exhausts.3,4 RE-doped ceria is very stableand exhibits high oxygen mobility. When a Ce4+ cationicsublattice gets substituted with a trivalent �RE� ion, oxygenvacancies are created in anionic sublattice in order to com-pensate the effective negative charge produced by the dopantcations.1,5 Geometry, structure, and properties of these de-fects resulting from true atomic substitution not only dependon the nature of the dopant, i.e., its crystal structure, elec-tronic charge, ionic radius, electronegativity, etc., but also onits mole fraction within the host matrix. This, in turn, in-creases oxygen diffusion within the host lattice and drasti-cally affects oxygen uptake and release by the system. Pure

ceria has fluorite structure �space group Fm3̄m� and formssolid solution with oxides of different rare earth elementsover a wide range of composition.6,7 In most cases, whenREO1.5 is doped into ceria lattice, phase changes from fluo-rite to C type with an intermediate biphasic region. Latticeparameter and other physical properties of the samples alsotend to change along with composition. It becomes evenmore exciting when the dopant ion is slightly bigger in sizethan the host Ce4+, as the incoming cation tries to expand thelattice, whereas the oxygen vacancies tend to shrink it. Thesetwo opposing factors accompanied by clustering of defects atdifferent sample compositions finally dictates differentphysical changes within the lattice that makes such studies avery interesting proposition. We have doped ceria with erbiaand holmia �both Ho3+ and Er3+ are bigger in size than Ce4+�

and studied their phase changes over very wide range of REconcentrations. X-ray diffraction �XRD� is the most widelyused technique for studying phase changes in crystalline ma-terials and has been extensively used in the present article.However, XRD is found to be insensitive to microscopicphase changes initiated by oxygen displacement, becausex-ray scattering efficiency of oxygen is significantly lowerthan that of the high-Z rare-earth cations. Moreover, owingto large coherence length of x rays, it is not often easy todetect parasitic microdomains of altogether different newphases amidst a bulk phase using an XRD technique.8 How-ever, such hidden information especially phase boundaries inhomogeneous samples can be of enormous importance forstructural materials that are proposed to be used for advancedapplications.6 The situation is further complicated when thedopant species exhibits very similar XRD patterns due tosimilarity in crystal structure. Micro-Raman spectroscopybecomes useful in such cases and promise better results be-cause of its sensitivity to both oxygen displacement due tolarge polarizability9 and intermediate range order withoutlong range periodicity.2,10 The technique has been efficientlyused as complementary to XRD in studying phase changes inCeO2−REO1.5 �RE=Er, Ho�. In the present article, we reportthe results obtained using both the techniques on phase rela-tions, and thermal expansion behavior of these two systems.

II. EXPERIMENTAL PROCEDURE

The starting materials, i.e., CeO2, Ho2O3, and Er2O3

were heated at 1073 K for 10 h to remove moisture and othervolatile impurities. About ten nominal compositions, each inthe CeO2−Er2O3 and CeO2−Ho2O3 systems were synthe-sized by a three-step heating protocol, wherein the powderswere thoroughly ground and heated at 1473 K for 36 h andthen at 1573 K for 36 h. In order to achieve better homoge-neity, the products obtained after the second heating wereagain ground, palletized, and heated at 1673 K for 48 h. The

a�Electronic mail: aktyagi@barc.ernet.in; Fax: 0091-22-2550 5151; Phone:0091-22-2559 5330.

JOURNAL OF APPLIED PHYSICS 103, 033506 �2008�

0021-8979/2008/103�3�/033506/7/$23.00 © 2008 American Institute of Physics103, 033506-1

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heating and cooling rate at every stage of heating was2 K /min and the atmosphere was static air. The starting ma-terials and all the nominal compositions prepared were char-acterized by powder XRD. XRD patterns were recorded on aPhilips x-ray diffractometer �Model PW 1710� with mono-chromatized Cu K� radiation. Silicon was used as an exter-nal standard for calibration of the instrument. In order todetermine the solubility limits, the lattice parameters wererefined by the least-squares method. For thermal expansionstudies, the XRD pattern of sample was recorded from 10° to90°, in temperature range 293-1273K on Philips X’pert ProXRD unit equipped with Anton Paar HTK attachment, at aregular interval of 200 K in static air. Raman spectra wererecorded using LABRAM–1, ISA make micro/macro-Ramanspectrometer. The 488 nm line of an Ar+ laser was used forexcitation and the scattered Raman signals were analyzed ina backscattering geometry using a single monochromatorspectrometer equipped with a Peltier-cooled charged coupleddevice �CCD� detector for multichannel detection.

III. RESULTS AND DISCUSSION

A. X-ray diffraction studies

Figure 1 exhibits the room temperature XRD patterns ofrepresentative samples from Ce1−xHoxO2−x/2 �0.0�x�1.0�series. It was observed that up to a nominal composition of40 mol % HoO1.5 in Ce1−xHoxO2−x/2 solid solution, sampleretained fluorite structure of the parent ceria as is evidencedfrom XRD patterns. Above this composition, i.e., at 45 and50 mol % of HoO1.5 in Ce1−xHoxO2−x/2, the peaks corre-sponding to C-type phase are too close to F-type phase andhence are difficult to resolve, whereas peaks at higher 2�value are slightly distorted �inset Fig. 1� indicating the pos-sible coexistence of two phases. However, at Ce0.5Ho0.5O1.75

composition, the C-type superstructure peak at 2��20.4 isvisible but other superstructure peaks are too small to beseen, which in turn, makes it extremely difficult for any

meaningful refinement of cell parameter using C-type lattice.This indicates that the sample could still be predominantly influorite structure along with a very little amount of C-typephase in it. The superstructure peaks increasingly grow inintensity as the content of HoO1.5 increases inCe1−xHoxO2−x/2 and the XRD peaks become more and morebroad and asymmetric indicating a biphasic field comprisingof both F-type and C-type lattices. At about 70 mol % ofHoO1.5 in CeO2−HoO1.5 series, C-type lattice dominates inthe biphasic mixture. From 80 mol % of HoO1.5 in CeO2

−HoO1.5 system, XRD patterns are almost the same as thatof HoO1.5, which exists in C-type structure. It may be addedthat XRD is not sufficient to delineate the exact phaseboundary in presence of microdomains of parasitic phase atboundary compositions.10,11 Thus Fig. 1 alone may not besufficient to comment at what critical concentration ofHoO1.5, the solution changes from pure F type to a mixtureof F and C types and then to single phasic C type. Previ-ously, researchers12,13 have shown that by quenching prod-ucts from 1400 °C, it is possible to induce a biphasic regionwithin ceria matrix at �48 mol % YO1.5 and the biphasicregion continues until 68 mol % of YO1.5. They also pre-dicted that the phase relation of CeO2−HoO1.5 should bevery similar to that of CeO2−YO1.5 system12,13 because ofsimilar ionic radii14 of Ho3+ and Y3+. Similar biphasic regionbetween 45 and 70 mol % of HoO1.5 has been observedfrom XRD studies on slow cooled samples in the presentinvestigation. Thus it seems, the quenching technique as re-ported previously12 might not have much significance over aslow cooled technique with regards to phase stabilization ofthe samples. It has been seen from Raman spectroscopy thatthe biphasic region actually starts from x=0.4, which is dis-cussed later. Figure 2 shows the typical XRD patterns ofrepresentative samples of the Ce1−xErxO2−x/2 �0.0�x�1.0�series. It is observed that F-type lattice is retained until

FIG. 1. Typical XRD patterns of �a� Ce0.9Ho0.1O1.95, �b� Ce0.55Ho0.45O1.775,�c� Ce0.5Ho0.5O1.75, �d� Ce0.4Ho0.6O1.7, �e� Ce0.3Ho0.7O1.65, and �f�Ce0.1Ho0.9O1.55. �Inset� Zoomed version of particular peaks are shown. �Anarrow in the inset helps to see the peak of the parasitic phase and the starmark represents the presence of superstructure peaks.�

FIG. 2. Typical XRD patterns of �a� Ce0.9Er0.1O1.95, �b� Ce0.55Er0.45O1.775, �c�Ce0.5Er0.5O1.75, �d� Ce0.4Er0.6O1.7, �e� Ce0.3Er0.7O1.65, and �f� Ce0.1Er0.9O1.55.�Inset� Zoomed version of particular peaks are shown. �An arrow in the insethelps to see the peak of the parasitic phase and the star mark represents thepresence of superstructure peaks.�

033506-2 Mandal et al. J. Appl. Phys. 103, 033506 �2008�

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40 mol % of ErO1.5, i.e., nominal composition ofCe0.6Er0.4O1.8. The biphasic region commences above thiscritical composition as is evidenced from the asymmetricbroadening of the XRD peaks �inset in Fig. 2� and it contin-ues until 70 mol % of ErO1.5. At and above nominal com-position of Ce0.2Er0.8O1.6, the solid solution exhibits pureC-type lattice.

The lattice parameters were calculated from x-ray dif-fraction patterns of individual samples from the two givenseries using POWDERX program and plotted in Fig. 3 as afunction of mole percent of REO1.5 in the ceria lattice. It maybe noted that lattice parameters of the samples exhibitingC-type lattice were multiplied by 0.5 so as to compare withthat of the F-type lattices. Moreover, insignificant contribu-tion from the superstructure peaks at the commencement ofbiphasic regions made refinement of the lattice parametersdifficult for certain representative samples. In general, ini-tially the lattice parameter decreases nonlinearly with in-creasing mole percent of REO1.5 until the biphasic regioncommences above 40 mol % of REO1.5 for both the series.The decrease in the lattice parameter with increasing REconcentration is observed in spite that both Ho3+ and Er3+

ions, in eightfold coordination, are marginally larger in sizethan the Ce4+ ion.14 This can only be attributed to the in-creasing O2− vacancies introduced into the system as a resultof incorporation of RE3+ in the ceria lattice. In the biphasicregion, the lattice parameter remains unchanged, and beyondthat tend to decrease again with increasing RE concentration.The increase in the C-type lattice parameter with increasingceria concentration in REO1.5−CeO2 �beyond the biphasicregion� may be attributed to increasing interstitial oxygenions in C-type lattice �Ho1−yCeyO1.5+y� that is expected toresult in dilation of the lattice due to mutual repulsion. Simi-lar observations were made by Bevan et al.12 for CeO2-Y2O3

series. In C-type �Ia3̄� sesque oxides two metal ions are notequivalent, their Wyckoff positions are 8b and 24d and oxy-gen ions occupy the 48e general position. The unoccupied16c position of this space group constitutes tetrahedral holesin C-type structure. The excess anions are placed in the 16csites.15 Bevan et al.12 proposed to fit the initial part �F type�of the curve �cell parameter versus compositions� with a qua-

dratic equation of the type a=A+B�RE+C�RE2, where adenotes the lattice parameter, RE denotes mole percent ofREO1.5 and A, B, and C are fitted parameters. According tothem, the term B is purely due to geometrical origin, whereasC originates from the strong attractive interaction betweenthe defects that tend to shrink the lattice. Accordingly wehave fitted the lattice parameter data in the region 0−40 mol % RE3+ for both series as explicitly shown in theinset of Fig. 3. The fitted parameters along with R2 valuesindicating goodness of the fit are given in Table I. It may benoted that the constant term A matches very well with thelattice parameter of pure CeO2 in both the cases. The B termis negative although it is expected to be positive if only“ionic size effect” is to be considered. Thus, a negative Bvalue clearly confirms the presence of oxygen vacancies inboth the series of samples. Moreover, the effect of the Cparameter is found to be much more pronounced in the caseof the CeO2−ErO1.5 series as compared to CeO2−HoO1.5

series, which point toward the defect clustering within thelattice.

In an attempt to explore possible temperature inducedphase changes, few representative samples from both the se-ries were investigated using high-temperature XRD tech-nique. However, in contrast to our previously reportedCeO2−GdO1.5 series16 wherein GdO1.5 undergoes high-temperature phase transition from C-type �cubic� to B-typemodification �monoclinic�, neither holmia nor erbia showedany phase transition for the temperature range �293–1273 K�studied. The stability under high-temperature �HT� of thecubic phase of CeO2−REO1.5 �where RE=Er, Ho� is be-lieved to have originated from the relatively smaller ionicsize17 of these RE3+. Temperature effect on dilation of latticeparameters was also studied using the same technique. Thelattice parameters were obtained from refinement of the insitu HT XRD data recorded on some typical samples and aregiven in Table II. The samples are tacitly chosen only fromthe monophasic regime so as to avoid ambiguity regardingdifferential lattice expansion of the two phases in the bipha-sic region. As expected, initially the lattice parameter in-creases almost linearly with temperature, but this incrementdeviates from linearity at higher temperatures due to contri-bution from the nonlinear terms. The linear thermal expan-sion coefficient ��� has been estimated for representativesamples from the mean variation of their lattice parametersaccording to the equation: �= ��a−a0� /a0�T�, where a0 de-notes lattice parameter at room temperature and �T is thedifference between the two temperatures and are also pro-vided in Table II. It is interesting to note that in either of thetwo cases, irrespective of F- and C-type lattices, thermalexpansion coefficient ��� decreases with increasing rare-

FIG. 3. Variation of lattice parameter with mole percent of REO1.5 �forRE=Er, Ho�. �Inset� Quadratic fitting of lattice variation for x=0−0.4.

TABLE I. Fitted parameters in quadratic variation of lattice parameters with%REO1.5.

Fitted parameters

Sample series A B C R2 values

Ce–Ho 5.411 −2.9�10−4 −5�10−6 0.994Ce–Er 5.411 −1.9�10−4 −1.3�10−5 0.998

033506-3 Mandal et al. J. Appl. Phys. 103, 033506 �2008�

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earth content, which may be attributed to higher meltingpoints of RE2O3 �RE=Ho, Er� as compared to that of CeO2.

B. Raman spectroscopic studies

Micro-Raman spectroscopy has been employed as acomplementary technique for investigating phase changes indoped ceria systems. Figure 4 shows the Raman spectra ofrepresentative samples from Ce1−xErxO2−x/2 �0.0�x�1.0�series. Pure ceria exhibits only a sharp first order Ramanpeak at �465 cm−1 assigned to the F2g mode due to sym-metrical stretching of the Ce–O vibrational unit in eightfoldcoordination.5 The F2g Raman peak ��465 cm−1� exists inthe range x=0−0.7, which establishes that F-type lattice isretained until x=0.7. Upon doping with ErO1.5, the transla-tional symmetry of the crystal is lost due to the introductionof defects and anion vacancies thereby slackening the k=0momentum conservation rule for Raman scattering. Hence,phonons from all parts of the Brillouin zone start contribut-ing to the optical spectra, thereby giving rise to severalbroadened, continuously spread, and weak-intensity bands.The principal peak at �465 cm−1 also gets broadened sig-nificantly accompanied by a decrease in intensity and at thesame time undergoes a subtle blueshift with increase in REconcentration. The apparent blueshift may be attributed tothe contraction of the lattice parameter as has been observed

from XRD studies and discussed previously �Sec. III A�. Wehave previously observed similar blueshift of the F2g Ramanband for YbO1.5- and TmO1.5-doped ceria systems.10 How-ever the observation is in contrast to that reported by Banerjiet al.18 for CeO2−GdO1.5 system wherein the F2g Ramanband actually undergoes a significant redshift and has beenexplained to be due to decrease in the average number ofoxygen atoms coordinated to the central cation that inducessome softening effect in the Ce–O vibrational mode. Irre-spective of the system studied, lattice contraction and soft-ening of Ce–O mode act in opposition to each other and theresultant of the two effects dictates the nature of shift ob-served for the F2g Raman band. In the case of the CeO2

−ErO1.5 system, the observed blueshift is only 3.5 cm−1.Above x=0.1, the F2g Raman band becomes highly asym-metric tailing toward higher wave numbers. The high-energytail may be associated with the emergence of a new peak at�488 cm−1 that tends to grow in intensity at the expense ofthe original peak at �465 cm−1 as x increases. Clearly, thepeak is in no way related to the commencement of the bi-phasic region as our XRD studies conclusively demonstratesa monotonic decrease in the lattice parameter with %RE inthat range as is expected within the monophasic F-type do-main. Moreover, no signature is observed for the principalRaman peak expected at �375 cm−1 for the C-type Er2O3.Banerji et al.18 reported similar high-frequency Raman bandat �482 cm−1 for the Ce1−xGdxO2−x/2 series and assignedthat to be due to vibrational modes of oxygen bonded to theGd atoms in GdO8 type of complex within the F-type lattice.In fact, such a broadened shoulder on the high-frequencyedge of the 465 cm−1 peak may be taken to be the signaturefor RE–O complexation within the fluorite host lattice. Thispossibly contributes to the high negative value of the C termin quadratic variation of the lattice parameter with %RE asdiscussed earlier. The apparent blueshift of the band by�6 cm−1 in the case of Ce1−xErxO2−x/2 as compared to thatreported for CeO2−Gd2O3 system may be attributed to thesmaller ionic size of Er3+ as compared to Gd3+. As this vi-brational mode is associated only with the movement of oxy-gen atoms, its frequency is not expected to have much ofdependence on the mass of the central cation as reportedpreviously.19 At x=0.3, the peak becomes highly asymmetricalong its lower energy flank indicating an extremely disor-dered structure, but still there was no visible signature forany peak around 375 cm−1 that could be assigned to the

TABLE II. High-temperature lattice parameters �Å� and linear thermal expansion coefficient of few represen-tative samples �within monophasic F and C regime� from CeO2−REO1.5 �RE=Er, Ho� series.

Ce1−xHoxO2−x/2 Ce1−xErxO2−x/2

T �K� x=0.1 x=0.2 x=0.8 x=0.9 x=0.1 x=0.2 x=0.8 0.9

293 5.407�1� 5.403�1� 10.652�2� 10.646�2� 5.406�1� 5.401�2� 10.622�2� 10.586�1�473 5.414�1� 5.410�3� 10.675�1� 10.671�3� 5.412�1� 5.414�3� 10.651�3� 10.601�1�673 5.427�1� 5.421�3� 10.692�2� 10.690�3� 5.425�1� 5.426�3� 10.668�2� 10.620�1�873 5.440�3� 5.423�1� 10.715�1� 10.713�3� 5.446�3� 5.440�3� 10.689�2� 10.639�1�

1073 5.456�1� 5.454�2� 10.736�2� 10.734�3� 5.464�1� 5.456�3� 10.709�4� 10.657�2�1273 5.467�1� 5.463�1� 10.757�1� 10.750�3� 5.465�1� 5.459�3� 10.728�2� 10.683�2�

��106 /K 11.38 11.33 10.06 9.97 11.13 10.96 10.18 9.35

FIG. 4. Raman spectra of erbia-doped ceria samples.

033506-4 Mandal et al. J. Appl. Phys. 103, 033506 �2008�

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C-type structure. It is only at and above x=0.4 that a clearhump could be observed at �375 cm−1 implying that thebiphasic region commences for x�0.4. This is in contrast tothe observations made from XRD studies wherein peaks re-lated to the C-type lattice appeared only from x=0.45 on-wards. Our previous studies,10 and similar studies by otherresearchers,11 have clearly established that such a discrep-ancy between Raman and XRD measurements as regards tothe commencement of biphasic region in an oxide mixturecould exist. Raman spectroscopy invariably indicates the on-set of biphasic region at a lower value of the dopant concen-tration because of its greater sensitivity toward oxygen dis-placement and lower coherence length that enables it todetect formation of microdomains of different phases.

As discussed previously, shift in F2g Raman band inErO1.5-doped CeO2 systems takes place due to the resultanteffect of oxygen vacancy and lattice contraction. But it is noteasy to delineate the two effects as proposed by McBride etal.,20 as oxygen vacancies also actively contribute to the ob-served change in the lattice parameter i.e., �a. In order toinvestigate the sole effect of lattice contraction on the shift ofthe Raman band, we have tacitly adopted in our present ar-ticle the theoretically calculated value of �a from Vegard’slaw as applicable within the monophasic F-type region �in-stead of observed change in lattice parameter�, wherefromwe obtained the Raman shift due to lattice dilation ���a�using the appropriate Grüneisen parameter ��=1.24� �Ref.21� in the following equation:

��a = − 3��0�a/a0, �1�

where �0 is the F2g peak and a0 is the lattice constant of pureceria. The data corrected for the Grüneisen shifts�F2g corrected� are then plotted as a function of mol %RE inFig. 5 along with the measured Raman frequencies for theF2g mode. The Grüneisen shifted peak positions show thesole effect of lattice contraction on the Raman shift and arealso plotted in the same graph for comparison. It may benoted that due to asymmetric nature of the curves for x=0.2 and 0.3, they were fitted with multi-Lorentzian lineshapes in order to extract the exact peak position for the F2g

mode. As shown in Fig. 5, the Grüneisen-corrected peak fre-quencies that depict the sole effect of oxygen vacancies onthe shift of the F2g Raman band increases monotonicallywith %RE for x=0−0.3. This clearly indicates that latticecontraction as brought about due to incorporation of oxygenvacancies in the CeO2−ErO1.5 lattice far exceeds the soften-ing effect due to lowering of average oxygen coordinationaround central cation, thereby resulting in the net blueshift ofthe measured F2g Raman band.

Apart from the principal Raman band at �465 cm−1,defect induced bands are also observed at �260 cm−1,560 cm−1 and 600 cm−1 for doped ceria systems. Defect-induced Raman spectra in Y2O3-, La2O3-, and ZrO2-dopedCeO2 have been extensively studied by Nakajima et al.8 Fol-lowing their work, the Raman band at �600 cm−1 is as-signed to defect spaces with Oh symmetry that include adopant cation in eightfold coordination of O2− but does notcontain any O2− vacancy �MO8-type complex�, whereas theRaman bands at 260 and 560 cm−1 are attributed to defectspaces that includes an O2− vacancy and have symmetry dif-ferent from that of Oh point group. The defect bands also getmodified as Er3+ content increases in the samples. The inten-sity of the Raman band at �260 cm−1 relative to that at�600 cm−1 i.e., I260 / I600 initially decreases from �1.2 to�0.9 and then remains nearly constant throughout the bipha-sic region. This implies that concentration of defects withoutan O2− vacancy, i.e., MO8 type, increases with %RE �prior tothe commencement of the biphasic region� as is also evi-denced from the emergence of the new peak at �488 cm−1.However, with the onset of biphasic region, the ratio remainsunchanged possibly because the C-type lattice formed in thebiphasic mixture does not accommodate either of these twotypes of point defects. Besides the peaks at �260 and540 cm−1 become significantly broadened and graduallyshift to higher wave numbers, the latter almost merging intothe peak at �600 cm−1, indicating large change in the sym-metry of the defect spaces above x=0.4 in Ce1−xErxO2−x/2.

Figure 6 shows the Raman spectra in the range200−1000 cm−1 of representative samples fromCe1−xHoxO2−x/2 �0.0�x�1.0� series segregated together indifferent phase regimes. Apart from the two end members,Raman spectra of all the intermediates are infested withstrong photoluminescence bands when excited at 488 nm,amidst which it is very difficult to track the modifications inthe weak embedded Raman signals as a function of %RE.These photoluminescence �PL� bands originating from f − ftransitions in Ho3+ are otherwise forbidden in pure Ho2O3.When incorporated in the host ceria lattice, these transitionsbecome allowed due to the lowering of symmetry around theHo3+ and their peak positions and line shapes keep on chang-ing with %Ho and also with changing phases as they areinvariably associated with changing symmetry around Ho3+.There is hardly any report that exploits PL interference inRaman spectrum to advantage in studying phase changes inRE-doped systems as has been done in the present treatise. Itis observed from Fig. 6 for x=0.1−0.3, PL spectra exhibits ahost of structured bands in the range 200−500 cm−1, theintensity that grows with the content of Ho3+. The increase inthe PL intensity as a function of mol% Ho3+ is an indicative

FIG. 5. Variation of peak frequencies vs mole percent of ErO1.5, of the F2g

Raman band for erbia-doped ceria samples �a� as measured, �b� Grüneisenshifted, and �c� corrected for Grüneisen shifts.

033506-5 Mandal et al. J. Appl. Phys. 103, 033506 �2008�

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of the fact that the overall symmetry around Ho3+ is main-tained and the samples still exist in the same phase, i.e., Ftype as predicted from XRD studies. At and above x=0.4, thePL bands get significantly broadened and the fine structureswithin these bands vanishes. Moreover, a new peak identifiedto be the Raman band arising out of defects associated withexcess interstitial oxygen in anion excess C-type�Ho1−yCeyO1.5+y/2� lattice emerges at 800 cm−1. Similar ob-servations have also been made for YbO1.5- andTmO1.5-doped ceria systems.10 The pattern continues to bethe same throughout the entire biphasic region. Here withincreasing %Ho the PL intensity decreases as the contribu-tion of the fluorite structure is reduced in the biphasic mix-ture, wherefrom those bands originate. However, as ex-pected, the peak at �800 cm−1 grows in intensity until x=0.9. Obviously, it is absent in pure HoO1.5. At and abovex=0.8, the PL bands get further broadened and less struc-tured, whereas the peak at �485 cm−1 identified to be theRaman band due to a MO8 type of defect clusters in theF-type lattice �also discussed in connection with CeO2

−ErO1.5 systems� vanishes indicating the onset of monopha-sic C-type regime as x=0.8.

IV. CONCLUSIONS

It has been demonstrated clearly using powder XRD andRaman spectroscopic techniques that ceria when sufficientlydoped with erbia and holmia undergoes a symmetry changefrom cubic F to C types via formation of biphasic mixture.The results obtained from XRD and Raman spectroscopytechniques are marginally different for Ce1−xErxO2−x/2. Al-though XRD studies predict that a biphasic mixture compris-ing of C- and F-type lattices is formed only at and above x=0.45, it has been confirmed from Raman spectroscopy thatthe C-type lattice is formed even in the sampleCe0.6Er0.4O1.8. The discrepancy is believed to have originatedbecause of the small domain size �smaller than the coherencelength of x rays� of the C-type lattice in Ce0.6Er0.4O1.8 inho-mogeneously distributed within the lattice. Different inter-mediate defective structures have been studied as a functionof sample composition in order to understand the drivingforce behind such phase transition. In the case of theCe1−xHoxO2−x/2 series, Raman spectra at excitation of 488nm is found to be infested with PL emissions that tend toovershadow the weak Raman bands. However, such an ap-parent shortcoming has been made use of to the advantage ofstudying phase changes in the system. It has been predictedfrom XRD measurements on Ce1−xHoxO2−x/2 that the bipha-sic regime commences at x=0.45 and it continues until x=0.7. PL measurements, however, shows that biphasic regionmight have started even before that sample composition i.e.,x=0.4. Below and above this critical composition band �x=0.4−0.7� samples exists in monophasic F- and C-type re-gimes.

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FIG. 6. Raman and PL spectra in the range 200−1000 cm−1 of holmia-doped ceria samples in �a� monophasic F-type regime, a=pure CeO2,b=10% HoO1.5, c=20% HoO1.5, and d=30% HoO1.5. �b� Biphasic regimecomprising of both F- and C-type lattices, e=40% HoO1.5,f =45% HoO1.5, g=50% HoO1.5, h=55% HoO1.5, i=60% HoO1.5, andj=70% HoO1.5. �c� Monophasic C-type regime, k=80% HoO1.5,l=90% HoO1.5, and m=pure HoO1.5.

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