DaltonTransactions

PAPER

Cite this: Dalton Trans., 2013, 42, 7672

Received 18th December 2012,Accepted 4th March 2013

DOI: 10.1039/c3dt33033h

www.rsc.org/dalton

An unusual temperature induced isostructural phasetransition in a scheelite, Li0.5Ce0.5MoO4†

Dipankar Saha,a Rajeev Ranjan,b Diptikanta Swain,c Chandrabhas Narayanac andTayur N. Guru Row*a

High resolution synchrotron X-ray diffraction, dielectric and Raman scattering study of a scheelite com-

pound Li0.5Ce0.5MoO4 (LCM) revealed that it transforms to a self similar structure above 400 °C. The ther-

mally induced isostructural phase transition (IPT), a phenomenon which has rarely been reported in the

literature, is preceded by partial softening of the zone centre phonons followed by their hardening

above the IPT transition temperature. The high temperature isostructural phase, which exhibits expanded

lattice parameters and cell volume, nucleates and grows in the low temperature matrix over a very wide

temperature range. Both the phases show nearly identical thermal expansion suggesting similarities in

symmetry, unaltered coordination environments around the atoms across the transition.

Introduction

Phase transitions in materials occur ubiquitously in natureand generally impart functionality to the system eventuallyleading to great technological innovations. Some of theseinclude materials depicting metal–insulator, ferroelectric–paraelectric, ferromagnetic–paramagnetic, normal–super-conducting transitions. On a microscopic scale, phase transitionsoriginate with the formation of small aggregates of atomsamounting to nano size nuclei which eventually grow in size.Commonly, structural phase transitions (SPTs) areaccompanied by change in crystallographic structure across acritical thermodynamic parameter (most often pressure andtemperature). Among the innumerable number of materialsexhibiting SPTs, there are very few instances in which the tran-sition is accompanied by neither change in the crystallo-graphic space group nor a change in occupied Wyckoffpositions. Such transitions have been categorized as “isostruc-tural phase transitions (IPTs), and have mostly been found by

mineralogists while investigating minerals, most of them withframework structures/large coordination environment arounda cation,1 under compression. IPTs are also known as“Cowley’s type zero” transition2 after Cowley postulated thepossibility of such a transition based on the behaviour ofacoustic phonons within the framework of displacive structuralphase transitions. Interestingly, temperature induced IPTs areamong the “rarest of rare IPTs” phenomenon reported so far.In this article we report an occurrence of one such phenom-enon in a complex scheelite Li0.5Ce0.5MoO4 using high resolu-tion synchrotron X-ray diffraction and Raman scatteringtechniques.

The first report of an IPT appeared in 1949 concerning Cemetal3,4 followed by reports of Cs,5,6 SmS,7 EuO,8 MnO,9

(V1−xCrx)2O3,10 ThAl2,

11 RFeO3,12 PbCrO3,

13 EuPd2Si2,14

TbPdAl,15 GdPdAl16 metallization in solid oxygen17,18 andmost recently in a iron–pictinide superconductor Na1−xFeAs.

19

In these systems IPTs have been associated with changes inthe electronic states and coordination environment around theatoms. IPTs have also been reported in systems which do notinvolve change in the electronic states, e.g. feldspars such asYbInCu4,

20 CsFeII[CrIII(CN)6],21,22 YbV4O8,

23 CaAl2Si2O8,24

NaAlSi3O8,25 carbonate minerals such as magnesite and dolo-

mite CaMg(CO3)226 and kaoline Al2Si2O5(OH)4.

27 IPT associ-ated with alteration in the coordination number (CN) has alsobeen reported in some inorganic compounds such as PbF2where CN increased from nine to ten in the high pressurephase,28 NH4PF6 –CN of the NH4 ion decreased from 14 to 12in the high temperature phase,29 CN increased from 9 to 11 inthe high pressure isostructural phase Na3MnF6,

30 sodiumoxalate.31 Apart from the bulk systems, very recently straininduced isostructural phase transition have been reported to

†Electronic supplementary information (ESI) available: Table S1 Intensity ratiosbetween peaks of the main phase and surrogate phase. Table S2 List of oxygencoordinates after Rietveld refinements. Fig. S1 Laboratory powder X-ray diffracto-gram of LCM. Fig. S2 Laboratory powder X-ray diffractogram of LCM in a narrow2θ interval at different temperature (a) Heating cycle (b) Cooling cycle. See DOI:10.1039/c3dt33033h

aSolid State and Structural Chemistry Unit, Indian Institute of Science,

Bangalore 560 012, India. E-mail: ssctng@sscu.iisc.ernet.in; Fax: +91-80-3601310;

Tel: +91-80-2292796bMaterials Engineering, Indian Institute of Science, Bangalore 560 012, IndiacLight Scattering Laboratory, Chemistry and Physics of Materials Unit,

Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O.,

Bangalore 560064, India

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occur in magnetoelectric BiFeO3 films,32,33 a system whichexhibits ferroelectric and magnetic ordering in the samephase. Christy1 has outlined possible mechanisms leading toIPTs in some of these framework structures under com-pression. Phenomenologically, IPT are required to be firstorder in nature. Surprisingly, majority of the reports on IPT arebased on pressure dependent studies and only in a few count-able cases temperature induced IPTs have been reported.29,34–39

In this article, we report the discovery of temperatureinduced IPT in a new class of compound, Li0.5Ce0.5MoO4

(LCM), belonging to scheelite family. This transition has beencharacterized by variable temperature synchrotron diffraction,Raman spectroscopy and dielectric measurements. Unlike allthe previous IPTs reported in the past, the IPT in the presentsystem is remarkable as it exhibits coexisting phases over awide range of temperature thus providing an ample proof forthe formation and subsequent growth of different nucleileading to separate seeds of the compound.

Experimental

Pure polycrystalline phase of Li0.5Ce0.5MoO4 (LCM) was syn-thesized following the method described in an earlier report.40

High-temperature X-ray diffraction studies were carried outusing Cu Kα X-ray source on a Bruker powder diffractometerwith a high temperature attachment (model: D8 Advance withmri TCPU1 high temperature system, Eurotherm temperaturecontroller). Diffraction data were collected from room tempera-ture to 650 °C at intervals of 50 °C. The samples were held for30 min at each temperature to equilibrate the change (±2 °C,Tantalum strip was used as heater) prior to recording. TheX-ray powder data were collected from 5–90° in 2θ with a stepsize of 0.02 with data collection time per step being 7 seconds.

Temperature dependent powder X-ray synchrotron datawere collected the Powder Diffraction Beam line (MCX)equipped with fast scintillator detector (Bede) at ELETTRA Syn-chrotron Light source, Trieste, Italy. The wavelength used forthe experiment was 1.0 Å, calibrated against Silicon(NIST-SRM660a) standards. The data were collected with a stepsize of 0.02° in the range 5–60°. Samples were loaded in acapillary after proper sonication to ensure uniform filling andcapillary was kept rotated during data accumulation. Tempera-ture of the sample was controlled by Hot blower heater (OxfordInstruments). The temperature of the sample was equilibratedfor 15 min prior to each data collection. Rietveld refinementswere carried out using software package FullProf.41

The temperature evolution of the Raman spectra of LCMwas recorded in the 180° back scattering geometry, using a532 nm excitation from a diode pumped frequency doubledNd-YAG solid state laser (model GDLM-5015 L, PhotopSuwtech Inc., China) and a custom-built Raman spectrometerequipped with a SPEX TRIAX 550 monochromator and a liquidnitrogen cooled CCD (Spectrum One with CCD 3000 controller,ISA Jobin Yovn-SPEX). Laser power at the sample was ∼8 mW,and a typical spectral acquisition time was 1–2 min. The

spectral resolution was 2 cm−1. The temperature was con-trolled with an accuracy of (0.1 K by using a temperature-con-troller (Linkam TMS 94) equipped with a heating stage unit(Linkam THMS 600). The spectral profile was fitted usingLorentzian functions with the appropriate background.

Dielectric measurements were carried out using pelletizedsamples using an impedance gain phase analyzer (NovocontrolAlpha-A) in the frequency range (100 Hz to 10 MHz (signalamplitude = 0.05 V) from (30–600) °C. Prior to measurement,the pellets were annealed at 700 °C for 1 h. Silver paste wasapplied on both sides of the pellet for better ohmic contact.Thin silver wires were soldered on both sides of the pellet. Thecoated pellets were heated at 150 °C in vacuum for 1 h toremove any organic component of the Ag-paste. Diameter andthickness of the pellet were approximately 10 mm and 1 mm,respectively.

Results and discussionVariable temperature X-ray powder diffraction studies

Previous structural studies have shown that Li0.5Ce0.5MoO4

(hereafter LCM) crystallizes in a space group I41/a with a struc-ture consisting of MoO4 tetrahedra linked to Ce/LiO8 poly-hedra.40 Fig. 1a shows the synchrotron X-ray powder diffractionpatterns of Li0.5Ce0.5MoO4 at 25 °C and 550 °C in a limited 2θrange. Appearance of additional weak reflections in thepattern of 550 °C (marked with arrows in Fig. 1(a)) suggeststhat the system exhibits structural phase transition (SPT) onheating. For the sake of clear visibility of the new weak reflec-tions, the intensity is shown on a logarithmic scale. These newreflections first become visible between 400 and 450 °C as canbe seen from the temperature evolution of the pattern inFig. 1(b). It is interesting to note that the overall intensity ofthe new peaks remain significantly smaller to those of themain peaks even at 550 °C, i.e. 150 °C above the transform-ation temperature.

Several plausible structural models were examined toexplore the appearance of additional weak reflections at hightemperature. Since the Bragg peaks corresponding to the roomtemperature phase are retained in intensity and the additionalpeaks are of a very low intensity, it was tempting to think ofthe structure of the high temperature phase to be a super-lattice of the low temperature phase, similar to those commonlyobserved in perovskites.42 Such transitions occur via smallreorientation of the octahedra and hence are displacive innature. The superlattice reflections in such cases are observeddue to a lowering of symmetry from the high symmetry parentcubic phase (space group Pm3̄m) on cooling below a criticaltemperature. In contrast to perovskites, in the present caseadditional reflections are observed on heating, rather than oncooling, above the critical temperature. A second model basedon a series of homologous compounds (Li1/2Ln1/2MoO4: Ln =La, Pr, Nd, Sm, Eu, Gd) also does not explain the observationmade in LCM as it represents an entirely different structuralmotif.43 Therefore, both these models were discarded. It is

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noteworthy that indexing using either strong peaks (i.e. ignor-ing all the new weak peaks) or using only the new weak peaks(ignoring the strong reflections), conformed to the tetragonalI41/a phase. For example at 550 °C, all the strong peaks couldbe indexed with a tetragonal lattice parameters a = 5.32527(1)Å, c = 11.73825(1) Å while all weak peaks yield a tetragonal cellwith a = 5.34223(1) Å, c = 11.80583(2) Å. Further the intensityratios between peaks belonging to either of these latticesremain invariant (See ESI Table S1†). Thus it becomes appar-ent that the most suited model would be to consider two inde-pendent self similar tetragonal phases to be associated witheach set of reflections (Strong reflection set and weak reflec-tion set). This was confirmed by fitting the high temperatureXRD pattern using two tetragonal phases by Rietveld structuralrefinement method (see Fig. 2). In the structural model theonly refinable parameters are the coordinates of the oxygenatom. Within the estimated standard deviation the

refinements reveal no significant changes in these coordinates(see ESI Table S2†). Further it is of interest to note that the setof new peaks, corresponding to the self similar structure,could be observed only in the high resolution synchrotrondiffraction experiments and were not noticeable when theexperiment was carried out using laboratory XRD. The temp-erature dependent lab XRD experiment (ESI Fig. S1†) thoughuseful in the beginning to suggest the existence of a phasetransition above 450 °C by revealing one new peak at 2θ =58.6° (ESI Fig. S2a†), failed to provide the minimum cluesbased on which a reliable analysis of the nature of the struc-tural phase transition could be ascertained.

Table 1 lists the phase fraction of the two tetragonal phasesand the R-factors for this fit. Two phase analysis was carriedout for the data at 450, 490 and 550 °C. Fig. 3 shows the vari-ation of the lattice parameters with temperature. In the twophase region, the a, c and V of the new tetragonal phase islarger by 0.02 Å, 0.07 Å and 0.32 Å,3 respectively. In terms ofpercentage these changes are ∼0.35%, 0.60% and 1.2%,respectively. It is interesting to note that the fraction of thehigh temperature tetragonal phase increases to only 15% at550 °C, i.e. even 100 °C above the IPT temperature (Inset ofFig. 3c), suggesting very sluggish nature of this IPT.

It is also of significance to note that the rate of increase ofthe lattice parameters a, c and unit cell volume (V) with temp-erature are identical for both the isostructural phases. Thisimplies that both the phases have nearly identical thermalexpansion coefficients. Since thermal expansion coefficient is

Fig. 1 (a) Showing appearance of weak peak left to each peak in room temp-erature phase. (b) Evolution of (0 0 4) peak with temperature.

Fig. 2 Fitted profile after Rietveld refinement with two identical tetragonalstructures of LCM at 550 °C.

Table 1 R factors and phase fractions for Rietveld refinement

Temperature Rp Rwp χ 2

Fraction (%)

Parent phase New phase

400 °C 8.31 11.67 2.68 96 4450 °C 6.71 9.07 2.88 91 9490 °C 6.67 8.68 2.54 90 10550 °C 8.71 12.31 5.69 83.23 16.77

Paper Dalton Transactions

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intimately related to the details of the crystal structure, it maybe conjectured that not only the structures are identical fromsymmetry point of view, but also that the atom coordination inboth the phases are more or less preserved. This feature is dis-tinctly different from the isostructural phases reported inseveral framework solids under pressure where the coordi-nation numbers are increased.36

Further, a careful observation of Fig. 1b reveals that thewidths of the Bragg peak is slightly reduced at 550 °C as com-pared to that at room temperature. Fig. 4 shows the tempera-ture variation of the full width at half maximum (FWHM) of a

representative Bragg peak at ∼29.5°. The FWHM increaseswith increasing temperature, remains nearly constant200–400 °C, and then decreases to its lowest minimum valueat 450 °C, the temperature at which the first signature of theformation of the isostructural high temperature phase wasobserved. Since the powders were obtained after crushingsingle crystals, the slight broadening of the peaks at roomtemperature cannot be attributed to crystallite size effect. Thepossibility of residual stress leading to increase in the FWHMat room temperature is also ruled out since one would expectthe FWHM to continuously decrease on heating (annealingeffect) instead of increasing in the temperature range50–400 °C as shown in Fig. 4. This broadening reappeared oncooling the system back to room temperature after heating upto 550 °C. It is therefore obvious that the initial increase in thewidth of the Bragg peaks is not due to extraneous factors butis a manifestation of precursor effect of the impending IPT. Itis likely that in the temperature range 50–400 °C, the enhancedFWHM is a manifestation of the stress build up in the host(low temperature) matrix as a result of nucleation of the iso-structural high T phase. The sudden drop in the FWHM at450 °C, is due to vanishing of the strain in the system after thegrowth of the high T phase in the matrix of the low T matrixwith well defined coherent interfaces between the lattices ofboth the phases.

Raman and dielectric studies

Raman study was performed to correlate the phonon behav-iour of LCM in relation to the observed IPT. The centrosym-metric LCM structure (space group I41/a) has two molecularformula units per primitive cell. In the standard notation,group theoretical considerations lead to 13 zone center Ramanactive modes: 3Ag + 5Bg + 5Eg.

44 Since the MoO42− tetrahedral

units of the structure are undistorted and independent, theRaman modes are generally interpreted as internal (stretching[ν1, ν3] and bending [ν2, ν4] modes) and external modes corre-sponding to rigid movements of the tetrahedra as rotation (R)and translation (T). The complete representation of the thir-teen Raman modes for the scheelite (LCM) can be describedas44

Γ ¼ ν1 ðAgÞ þ ν2 ðAgÞ þ ν2 ðBgÞ þ ν3 ðBgÞ þ ν3 ðEgÞ þ ν4 ðBgÞþ ν4 ðEgÞ þ R ðAgÞ þ R ðEgÞ þ 2 T ðBgÞ þ 2 T ðEgÞ:

Fig. 5 shows the temperature dependent Raman spectra ofLCM in the range 150–1050 cm−1. The modes have beenassigned based on an earlier Raman study for an analogouscompound Na0.5La0.5MoO4.

38 The strong modes at ∼900 cm−1

and ∼300 cm−1 are assigned as ν1 (Ag) and ν2 (Ag) + ν2 (Bg)respectively. The intense Raman band centred at 890 cm−1 rep-resents the ν1 mode whereas ν3 modes appear around 750, 790and 824 cm−1, ν4 at 382 cm−1 and ν2 at 333 and 312 cm−1. Themodes at 250 cm−1 and 194 cm−1 are associated with libra-tional motion of MoO4 tetrahedra and translational motionassociated with Li1+/Ce3+ ion, respectively.45 Interestingly, the

Fig. 3 Variation of cell parameters and volume with temperature (Inset showsthe volume fraction of room temperature phase with temperature).

Fig. 4 Full width half maxima (FWHM) analysis of LCM.

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mode around 250 cm−1 broadens considerably with tempera-ture and merges with background at 300 °C, i.e. well below thephase transformation temperature (∼450 °C). Li-modifiedScheelite compounds have been reported to exhibit Li+ ionmobility at elevated temperatures.46,47 Though not verifieddirectly, dielectric measurements of LCM, shown in Fig. 6, alsoshows considerable increase in the conductivity with tempera-ture, presumably due to the increased Li mobility.40 The loss

tangent also exhibits a small hump at ∼450 °C (Fig. 6) at theIPT transition temperature.

The gradual vanishing of the mode at 250 cm−1 is attribu-ted to the increasing dynamic disorder in the librationalmotion of the MoO4 tetrahedra coupled to the Li/Ce motion.For analysis related to the IPT, we have considered only thewell resolved ν1, ν2 and ν4 modes. The three ν3 modes in therange 750–825 cm−1 are difficult to resolve above 150 °C. TheRaman spectra of LCM reveal the following features aroundthe IPT: (a) relatively faster decrease in the frequency of theν1 and ν4 modes as the IPT temperature is approached fromroom temperature, (b) stiffening of the modes above the tran-sition temperature, (c) appearance of a new mode at∼440 cm−1, and (d) anomalous increase in the width of theRaman modes (a typical representative plot is given in Fig. 7(a)). The results suggest that similar to other displacive SPTsinvolving symmetry change, the IPT in LCM is also driven byphonon softening mechanism. It is known that partial

Fig. 5 Temperature evolution of the Raman spectra of Li0.5Ce0.5MoO4.

Fig. 6 Temperature dependence of the relative permittivity and loss tangent ofLi0.5Ce0.5MoO4 at 10 kHz.

Fig. 7 Temperature dependence of (a) Symmetric stretching modes of theMoO4 ion (Inset: FWHM of the ν1 mode of the MoO4 ion). (b) Bending modes.(c) Bending modes and appearance of new bending mode in the temperatureinterval 25–600 °C.

Paper Dalton Transactions

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softening of modes can as well lead to SPT. In the present casemore than one mode softens to result in an IPT.

Since the high temperature phase is isostructural to thelow-temperature phase with expanded lattice parameters, it isexpected that the Raman modes of the high temperaturephase should be similar to that of the low temperature phasewith slight change in the frequency due to difference in thelattice parameters. Since the lattice parameters of the new iso-structural phase are not very drastically different, and also thevolume fraction of this phase is significantly low even at550 °C, the Raman peaks corresponding to the high tempera-ture isostructural phase would not be visible as a set of separ-ate peaks in the pattern. They would rather be superimposedon the spectra of the low temperature phase, as evident by theincrease in the width of the modes above the IPT temperature.The evolution of the Raman spectra with temperature, shownin Fig. 5, is consistent with the IPT nature of the transition.Given the fact that the lattice parameters and the cell volumeof both the coexisting isostructural phases differ by ∼0.5 and1%, respectively, a local lowering of symmetry is expectedaround the interface of the two phases to accommodate thestrain induced due to the lattice mismatch. The new modeobserved at ∼440 cm−1 could be either due to appearance ofthe one of the symmetries (Bg and Eg) of the ν4 mode or due tolifting of the degeneracy of the two Eg modes due to local low-ering of symmetry near the interface regions.

Conclusions

Temperature dependent high resolution synchrotron X-raypowder diffraction accompanied by Raman and dielectricstudies unambiguously establish this temperature induced IPTin Li0.5Ce0.5MoO4. In contrast to the previously reported IPTs,this temperature induced IPT is uncommon and found to bedriven by nucleation and growth of the new self similar phasein the matrix of the room temperature phase over a wide rangeof temperatures. The observation of a slight softening of someof the zone centre phonons points to the displacive nature ofthis IPT. It may be anticipated that temperature induced IPTmight occur more frequently, particularly in complex non stoi-chiometric oxides. It is worthwhile to explore a special featureby variable temperature synchrotron XRD data analysis inorder to unearth such IPTs in inorganic oxides.

Acknowledgements

Dipankar thanks Lalithalakshmi B. V. for high temperaturelaboratory PXRD measurement and Lalitha K. V. for dielectricmeasurements and DST-India and Sincrotrone Trieste forfinancial support during the synchrotron data collection atTrieste, Italy. Dipankar also thanks Dr Jasper Plaisier for scien-tific assistance during the synchrotron data collection. TNGthanks DST for J. C. Bose fellowship for funding.

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Paper Dalton Transactions

7678 | Dalton Trans., 2013, 42, 7672–7678 This journal is © The Royal Society of Chemistry 2013

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