ARTICLE IN PRESS

0921-4526/$ - se

doi:10.1016/j.ph

�Correspondifax: 0910731246

E-mail addre

Physica B 365 (2005) 47–54

www.elsevier.com/locate/physb

Synthesis and investigation of structural and electronicproperties of Pr1�xCaxFeO3 (0pxp0.2) compounds

S.K. Pandey�, R. Bindu, Pramod Bhatt, S.M. Chaudhari, A.V. Pimpale

UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452017, India

Received 30 November 2004; received in revised form 7 March 2005; accepted 28 April 2005

Abstract

Single-phase powder compounds of Pr1�xCaxFeO3 (x ¼ 0, 0.1 and 0.2) have been synthesized by combustion method

using as starting material the corresponding metal nitrates and glycine. The compounds were characterized by X-ray

diffraction (XRD), Fe K-edge X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) of

core level of Pr 3d, Fe 2p, Ca 2p and O 1s. Rietveld profile refinement technique was employed to analyze the powder

diffraction data. All the three compounds showed orthorhombic structure with systematic reduction in lattice

parameters. The distortion in FeO6 octahedra also reduced as x increased. The chemical shift of Fe K-edge showed that

the Fe is in 3+ state in PrFeO3 and its effective charge increased on Ca doping. XPS studies revealed that the ionic state

of Pr, Fe and Ca is close to the ionic state of these elements in Pr2O3, Fe2O3 and CaO, respectively. On substituting Ca

for Pr, effective charge on Pr and Fe increased whereas that on O decreased. Change in exchange splitting consistent

with the increased effective charge was also observed on Ca doping.

r 2005 Elsevier B.V. All rights reserved.

PACS: 61.10.N; 78.70.D; 79.60

Keywords: X-ray diffraction; X-ray absorption spectroscopy; X-ray photoelectron spectroscopy

1. Introduction

Perovskite-type transition metal oxides havebeen of much interest for more than fifty years.Recently, research activities have been intensifiedin this class of materials due to the emergence of

e front matter r 2005 Elsevier B.V. All rights reserve

ysb.2005.04.036

ng author. Tel.: 09107312463913;

5437.

ss: sk_iuc@rediffmail.com (S.K. Pandey).

exotic properties like charge disproportionation,charge ordering (CO), orbital ordering (OO),phase separation, colossal magneto resistance(CMR), etc. [1]. The interplay between the on siteand inter site Coulomb interaction, the chargetransfer energy, the hybridization strength be-tween the cation 3d and oxygen 2p states and thecrystal field splitting for the dnpm configuration ofthe MO6 (M ¼ 3d transition metal ion) octahedracontrol the ground state electronic structure,

d.

ARTICLE IN PRESS

S.K. Pandey et al. / Physica B 365 (2005) 47–5448

magnetic and transport properties of these per-ovskites. The compounds under study, Pr1�xCax-

FeO3, also show above interesting properties forspecific ranges of x. Doping of Ca at Pr-site causestwo fold effects: (1) to vary the number ofelectrons (band filling) and hence change theelectronic configuration, and (2) the size effect,which changes the inter-atomic distances and bondangles. Depending upon the amount of chemicalpressure introduced into the lattice, changes occurin the local environment and later on extendthroughout the lattice. These changes are reflectedin the Fe–O bond distances and Fe–O–Fe bondangles, which play crucial role in governing thephysical properties. At room temperature both theend members of the series, i.e. PrFeO3 andCaFeO3, are orthorhombic with PrFeO3 beingantiferromagnetic insulator and CaFeO3 metallicand paramagnetic. CaFeO3 shows metal–insulatortransitions near 290K [2]. It also has a spiralantiferromagnetic spin structure with Neel tem-perature 115K [3]. Mossbauer studies on thiscompound at 4.2K reveal the presence of twochemically distinct sites with different hyperfinefields present in equal proportions [4]. Thisobservation has led to the widely held view thatthe system exhibits charge disproportionation ofthe type 2Fe4+ (d4)-Fe3+ (d5)+Fe5+ (d3). Inthis compound charge disproportionation is alsoaccompanied by real space ordering of Fe3+ andFe5+ ions, known as CO [5].

In the related compounds of the type AFeO3

with A standing for La and/or Sr, electronicstructure is relatively better known; however, forA ¼ Pr it is not so very well known. LaFeO3 isionic insulator with large band gap. When Sr isdoped at the La sites the resulting holes mainlyoccupy O sites and doped holes have relativelypure p-hole character, as demonstrated by O 1sX-ray absorption studies [6]. On the other hand,SrFeO3 is strongly covalent and the band gapdisappears making it metallic. The ground state ofSrFeO3 is dominated by the d5L�1 configurationconsisting of 5 electrons in Fe 3d orbitals and onehole in O 2p orbital rather than the d4 configura-tion [7].

It seems that very little work on transport,structure, magnetic and electronic properties has

been done on Pr1�xCaxFeO3. Perhaps this isbecause making these compounds by standardsolid-state route is difficult, as one has to applyvery high pressure (�GPa) and it also takes muchtime and energy.We have successfully applied the combustion

route to synthesize Pr1�xCaxFeO3 (x ¼ 0, 0.1 and0.2) compounds. To the best of our knowledge thisis for the first time this method has been appliedfor making these compounds. The compoundswere characterized by using powder X-ray diffrac-tion (XRD) and details of their structures havebeen analysed by using Rietveld refinementtechnique. Electronic states were studied by usingFe K-edge X-ray absorption spectroscopy (XAS)and X-ray photoemission spectroscopy (XPS).Using XPS technique we have studied the 2p3/2and 2p1/2 states of Fe and Ca, 3d5/2 and 3d3/2 statesof Pr and 1s1/2 state of oxygen. All the threecompounds are orthorhombic and systematicvariations in the lattice parameters and atomicpositions are observed. Fe K-edge XAS studiesshow that iron is in Fe3+ state in PrFeO3 andwhen Ca is doped at Pr-site the effective charge onFe increases. This is also consistent with Fe 2pcore level studies by XPS. Pr 3d state XPS studiesalso indicate the increment in effective charge onPr ion whereas the effective charge on O iondecreases on Ca doping.

2. Experimental

Powdered samples of Pr1�xCaxFeO3 (x ¼ 0, 0.1and 0.2) were prepared by combustion method [8].Nitrates of Fe, Ca and Pr were taken in appro-priate amount and mixed in double distilled water.In this mixture glycine, 2 moles of glycine per1 mole of metal cation, was added and stirred untilall compounds dissolved in water. The resultingsolution was heated slowly at temperature around200 1C till all the water evaporated. The precursorthus formed catches fire on its own making thepowder of the desired compound. For the parentcompound PrFeO3, single-phase polycrystallinecompound was obtained in this manner. However,for x ¼ 0:1 and 0.2 compounds additional heattreatment for one day was required at 600 1C.

ARTICLE IN PRESS

S.K. Pandey et al. / Physica B 365 (2005) 47–54 49

The XRD patterns were recorded with mono-chromatized Cu–Ka radiation in the 2y range of101–901 using Rigaku powder X-ray diffracto-meter. The rotating anode X-ray generator wasoperated at 40 kV and 100mA. The mono-chromator used was graphite (0 0 2) and the widthsof the divergent slit, scattering slit and thereceiving slit were 0.51, 0.51, 0.15mm, respectively.The data were collected with a step size of 0.021with a scanning rate of 21/min. The XRD pat-terns were fitted using Rietveld profile refinementtechnique [9].

X-ray absorption spectra were recorded byfollowing the usual procedure of putting finepowder on adhesive tape and taking an appro-priate number of such tapes to optimize the samplethickness. 12 kW Rigaku rotating anode X-raygenerator was used with copper target. Thegenerator was operated at 18 kV and 150mA.Johannson bent crystal spectrometer was used forrecording the absorption spectrum [10]. Themonochromator crystal used was Si (3 1 1) withNaI (Tl) scintillation detector. The sample wasoscillated in and out of the beam path to recordthe transmitted and incident intensities at roomtemperature, 300K. The receiving slit was 100 mand it took about 4 h to record the absorptionspectrum over the energy range from 30 eV belowthe Fe K-edge to 150 eV above it. To check theresolution, FWHM at Cu–Ka1 was measured andit was found to be 6 eV. Step size for the datacollection was given as 2y ¼ 0.011 and experimen-tal accuracy was about 1 eV. For calibration andcomparison purposes reference spectra of Fe andFe2O3 were also recorded. All the spectra werenormalized to an edge height of 1.0 at about 50 eVabove the edge.

The XPS measurements were carried out usingphotoelectron spectrometer equipped with anOMICRON electron analyzer (model EA125).All the XPS measurements reported in the presentwork were carried out using Mg–Ka radiation at50 eV pass energy of the spectrometer. Themeasured resolution was about 0.85 eV withinstrumental accuracy within 0.2 eV. All thecompounds were pressed into hard pellets andsintered at 900 1C for 24 h; it may be mentionedhere that this additional heat treatment does not

show any change in the XRD patterns for any ofthe three compounds. Thus heating these com-pounds at elevated temperatures does not lead toany change in their crystal structures. The pressedsample was then mounted on sample holder usingsilver paste as fixing agent and to assure goodelectrical contact with spectrometer. It was scrapeduniformly by diamond file before carrying out themeasurements. Final spectrum was taken onlywhen the feature coming from carbon contamina-tion of the surface merged into the spectrumbackground. Vacuum of the chamber during theexperiment was �10�9 Torr.

3. Results and discussions

3.1. Structural studies

The XRD patterns for all the three compoundswith fitted patterns are shown in Fig (1). In all thethree compounds we found the same number ofpeaks. For all the compounds the main peak arisesaround 2Y ¼ 32.61 and doublets are also observedaround 46.41 and 46.61. These peaks shift towardshigher 2Y as x increases. For the doublet, the first(at lower 2Y) peak is higher in intensity and theratio of the intensities of the two peaks decreasesas Ca content increases.The XRD patterns of all the samples were fitted

using Rietveld profile refinement software [9]. Theparent compound PrFeO3 shows orthorhombicstructure with Pbnm space group. The ortho-rhombic unit cell contains 4 formula units. Thestructure for x ¼ 0 compound—PrFeO3—isavailable in the literature [11,12]. These data wereused as initial parameters for fitting the pattern.Our refined lattice parameters are close to thereported ones, Table 1. Remaining two com-pounds were also fitted with Pbnm space group.The Rietveld fitting for all the compounds areshown in Fig (1). Typically, our goodness of fitgiven by the parameter S is the ratio of weightedpattern and expected pattern (Rwp=Re) and isaround 1.2. The atomic positions for orthorhom-bic structure (Pbnm) are Pr/Ca: 4c(x,y,1/4), Fe:

4b (0,1/2,0), O(1): 4c(x,y,1/4), O(2): 8d (x,y,z).Reitveld refined position of atoms in the unit cell

ARTICLE IN PRESS

S.K. Pandey et al. / Physica B 365 (2005) 47–5450

are also listed in Table 2 for all the compounds.Bond distances and bond angles for these com-pounds were calculated using powder cell software[13] and are listed in Table 3.

Table 1

Lattice parameters of Pr1�xCaxFeO3 (x ¼ 0, 0.1 and 0.2) at room tem

Pr1�xCaxFeO3 Cell parameters (A) [12] Cel

PrFeO3 a ¼ 5.4820 a ¼

b ¼ 5.5780 b ¼

c ¼ 7.7860 c ¼

Pr0.9Ca0.1FeO3 a ¼

— b ¼

c ¼

Pr0.8Ca0.2FeO3 a ¼

— b ¼

c ¼

Fig. 1. Rietveld profile fitting for the XRD patterns of

Pr1�xCaxFeO3 (x ¼ 0, 0.1 and 0.2). The observed and the

calculated patterns are represented by hollow circles and solid

line, respectively.

On replacing Pr partially by Ca it is expectedthat some of the Fe3+ will be converted to Fe4+ tomaintain charge neutrality. Here it is implicitlyassumed that doped holes only reside on the Fesite. However, in general, this may not be true. Thetolerance factor for perovskite structure given byt ¼ ðrA þ rOÞ=½

p2ðrB þ rOÞ], where ri is the ionic

radius of the ith ion, also plays a role in structure.For an ideal perovskite t is unity. Assuming that inPrFeO3 Pr is in 3+ state, Fe in 3+ state and Oin 2� state and employing the ionic radii fromliterature [14], the value of t is 0.909. For CaFeO3,with Ca in 2+ state and Fe in 4+ state the valueof t is 0.921. Thus CaFeO3 is closer to idealperovskite and one expects decrease in latticedistortion on replacing some amount of Pr by Ca.

perature obtained from Rietveld fitting. S is the goodness of fit

l parameters (A) c/O2a S ¼ ðRwp=ReÞ

5.4832(2)

5.5722(2) 1.004 1.21

7.7866(3)

5.4773(3)

5.5519(3) 1.003 1.15

7.7735(4)

5.4658(4)

5.5236(4) 1.002 1.12

7.7504(6)

Table 2

Atomic positions of different atoms in the unit cell of

Pr1�xCaxFeO3 (x ¼ 0, 0.1 and 0.2) compounds obtained from

Rietveld fitting

Pr1�xCaxFeO3 Pr/Ca Fe O1 O2

PrFeO3 X ¼ 0.9902(4) 0.0 0.0924(28) 0.7103(23)

Y ¼ 0.0436(2) 0.5 0.4772(25) 0.2924(21)

Z ¼ 0.25 0.0 0.25 0.0423(16)

Pr0.9Ca0.1FeO3 X ¼ 0.9912(4) 0.0 0.0920(31) 0.7121(24)

Y ¼ 0.0414(2) 0.5 0.4714(24) 0.2955(22)

Z ¼ 0.25 0.0 0.25 0.0396(18)

Pr0.8Ca0.2FeO3 X ¼ 0.9906(5) 0.0 0.0860(36) 0.7085(26)

Y ¼ 0.0386(3) 0.5 0.4749(24) 0.2937(24)

Z ¼ 0.25 0.0 0.25 0.0352(20)

ARTICLE IN PRESS

Table 3

Bond distances Fe–O(1) (A), Fe–O(2) (A), bond angles Fe–O(1)–Fe (deg), Fe–O(2)–Fe (deg) and volume of the unit cell (A3)

Pr1�xCaxFeO3 Fe–O(1) (A) Fe–O(2) (A) Fe–O(2) (A) Fe–O(1)–Fe (deg) Fe–O(2)–Fe (deg) Volume (A3)

PrFeO3 2.015(4) 1.992(14) 2.023(18) 149.96(94) 153.52(47) 237.91(1)

Pr0.9Ca0.1FeO3 2.013(3) 1.967(15) 2.033(19) 149.58(68) 154.12(52) 236.38(2)

Pr0.8Ca0.2FeO3 1.998(4) 1.977(16) 2.001(21) 151.61(85) 155.09(53) 233.99(3)

The atomic positions for orthorhombic structure (Pbnm): Pr/Ca: 4c(x,y,1/4), Fe: 4b (0,1/2,0), O(1): 4c(x,y,1/4), O(2): 8d (x,y,z)

Fig. 2. Normalized Fe K-edge X-ray absorption spectra of Fe,

Fe2O3 and Pr1�xCaxFeO3 (x ¼ 0, 0.1 and 0.2).

Table 4

Edge positions and chemical shifts (eV) of Fe, Fe2O3 and

Pr1�xCaxFeO3 (x ¼ 0, 0.1 and 0.2)

Compounds Edge positions (eV) Chemical shifts (eV)

Fe 7111 —

Fe2O3 7120.5 9.5

PrFeO3 7120.6 9.6

Pr0.9Ca0.1FeO3 7123.8 12.8

Pr0.8Ca0.2FeO3 7124.1 13.1

S.K. Pandey et al. / Physica B 365 (2005) 47–54 51

For the x ¼ 0:1 and 0.2 compounds the structureremains orthorhombic. With increase in Ca dop-ing, the c=

p2a tends to unity indicating reduction

in the distortion of FeO6 octahedra. When thisfactor is exactly unity the diffraction peakscorresponding to [2 2 0] and [0 0 4] reflections willmerge. Moreover, the shift in [2 0 0], [0 2 0] and[0 0 2] peaks towards higher 2Y with increase in x

indicates reduction in the value of lattice para-meters. This indicates reduction in the unit cellvolume, Table 3. Regarding Fe–O bond distanceswe observe decrease in apical Fe–O(1) bonddistances whereas there is no variation in Fe–Obond distances in the basal plane (i.e. Fe–O(2))within the error bar. Thus the FeO6 octahedroncontracts along the c-axis. The Fe–O(1)–Fe andFe–O(2)–Fe bond angles also shift to 1801 withdoping. This further substantiates the decrementof distortion with hole doping.

3.2. X-ray absorption spectroscopy

Normalized Fe K-edge X-ray absorption spec-tra, edge positions and chemical shifts for thethree compounds as well as the two standardcompounds (Fe and Fe2O3) for calibration andcomparison are shown in Fig (2) and Table 4.Chemical shift is estimated with respect to theedge position of Fe powder. The spectrometerwas calibrated using the K-edge of Fe powder at7111.0 eV [15]. For all the compounds, thepoint corresponding to the half of the edge heightof mx is taken as the edge position (i.e. thresholdenergy). All the spectra were normalized foredge height of unity at 50 eV above the edge.Our edge position for Fe2O3 matches with theedge position of Grunes [15] within 4.7 eV of

ARTICLE IN PRESS

S.K. Pandey et al. / Physica B 365 (2005) 47–5452

accuracy. Edge positions of Fe2O3 and PrFeO3 areclose indicating the fact that the ionic state ofFe atom in PrFeO3 is the same as in Fe2O3

which is taken as Fe3+. Moreover, on replacing10% Pr atoms by Ca atoms the edge positionis shifted to higher energy, Table 4. This indicatesthat the effective nuclear charge of Fe increaseson Ca doping. On replacing another 10% of Pratoms by Ca atoms, we observed no appreciableshift in edge position within experimental accu-racy. This result is consistent with XPS resultswhere no appreciable change in Fe 2p states isobserved.

3.3. X-ray photoemission spectroscopy

Core level XPS spectra for Pr, Fe, O and Ca areshown in Fig (3). Peak positions of carbon (C) 1s1/2 for x ¼ 0, 0.1 and 0.2 compounds before scrapingare observed at 293.6, 285.6 and 285.1 eV,respectively. These shifts in binding energy (BE)are due to charging of the compounds as they areinsulating. To compensate this effect, all thespectra of x ¼ 0, 0.1and 0.2 compounds are shiftedby �9.0, �1.0 and �0.5 eV, respectively, as the BEof C 1s1/2 in the metallic compounds is 284.6 eV.To identify the peak positions (i.e. BE) of the corelevels in better way we have taken the secondderivatives of the spectra. BE of Pr 3d, Fe 2p, Ca2p and O 1s are listed in Table 5. Exchangesplitting of all the core levels except O 1s is alsogiven in Table 6.

The BE of Fe 2p3/2 state in PrFeO3 is 708.7 eV.Bouquet et al. [7] have also reported the BE of Fe2p3/2 state in Fe2O3, LaFeO3 and SrFeO3

compounds at 711.2, 709.4 and 709.3 eV,respectively. Thus chemical shift of �0.7 eV isobserved in PrFeO3 with respect to LaFeO3.The exchange splitting— a measure of theelectron-spin interaction— is 14.0 eV for Fe 2p inPrFeO3, which is about 0.8 eV more than that inFe2O3. On replacing Pr atoms by Ca atoms theBE of Fe 2p3/2 state increases. This behaviourindicates that the effective charge on Fe ionincreases on doping. Chainani et al. [16] have alsoobserved similar shift in BE of Fe 2p3/2 forLa1�xSrxFeO3 compounds. However, there isdecrement in exchange splitting of Fe 2p state on

Ca doping. This is mainly because the Fe 2p3/2state is pulled more towards nucleus than theFe 2p1/2 state on hole doping.The BE of Pr 3d5/2 state in PrFeO3 is 931.7 eV.

This BE is smaller than the BE of Pr 3d5/2 state inPr2O3 and PrO2 [17]. In PrFeO3 exchange splittingof Pr 3d orbital is found to be 20.3 eV. Onreplacing 10% of Pr by Ca, BE of 3d5/2 stateincreases by 0.4 eV whereas exchange splittingincreases by 0.2 eV, which is well within experi-mental accuracy. On further replacing of Pr by Ca,in the compound Pr0.8Ca0.2FeO3 BE of 3d5/2 stateincreases only by 0.1 eV whereas the exchangesplitting increases by 0.8 eV. This occurs because3d3/2 state is attracted more towards the nucleusthan the 3d5/2 on hole doping, Table 5. It may benoted that the lattice parameters as revealed byXRD change systematically with doping whereasour data show that the electronic state of theatoms occupying rare-earth sites with lower j-valueare affected most. This may be because XRD givesinformation about the integrated intensity, whichcomes from elastic scattering of photons from allthe atoms. It is not necessary that this effect beequally reflected in all the states of the atomsprobed by XPS and XAS as these techniquesreveal information about specific electronic states.In addition to this it is known that the measuredbinding energies for different states are sensitive tofinal state screening effect, polarization andrelaxation energy. These effects are different incompounds with different compositions. It isgenerally difficult to separate out their contribu-tions from XPS measurements in the absence ofdetailed theoretical calculations.Next we compare the BE of O 1s1/2 state in

Fe2O3 and PrFeO3 Table 5. Chemical shift of�1.1 eV is observed for PrFeO3. Bocquet et al. [7]have also observed the BE of O 1s1/2 electronsat 528.7 eV in LaFeO3. On doping 10% of Ca, BEof 1s1/2 electrons decreases by 1.0 eV. Thisbehaviour indicates the decrease in the effectivecharge on O ions. The BE of Ca 2p3/2 in x ¼ 0:1compound is 345.7 eV whereas in CaO it is347.3 eV [18]. In this compound the exchangesplit of 4.3 eV is observed whereas in CaO it is3.6 eV. On 10% Ca doping in x ¼ 0:1 compoundBE of Ca 2p3/2 increases only by 0.2 eV whereas

ARTICLE IN PRESS

Fig. 3. Core level X-ray photoemission spectra of Pr 3d, Fe 2p, Ca 2p and O 1s.

Table 5

Experimental binding energies (eV) for XPS Core-level main peaks of Pr1�xCaxFeO3 (x ¼ 0, 0.1 and 0.2) and standard compounds

Core-levels x ¼ 0 x ¼ 0.1 x ¼ 0.2 LaFeO3 [7] Fe2O3 [19] Pr2O3 [17] PrO2 [17] CaO [18]

Pr 3d5/2 931.7 932.1 932.2 — — 933.2 935.3 —

Pr 3d3/2 952.0 952.6 953.5 — — — — —

Fe 2p3/2 708.7 709.4 709.6 709.4 710.8 — — —

Fe 2p1/2 722.7 723.2 723.2 — 724.0 — — —

Ca 2p3/2 — 345.7 345.9 — — — — 347.3

Ca 2p1/2 — 350.0 350.7 — — — — 350.9

O 1s1/2 529.1 528.1 528.3 528.7 530.2 — — —

S.K. Pandey et al. / Physica B 365 (2005) 47–54 53

ARTICLE IN PRESS

Table 6

Experimental exchange splitting (eV) of XPS core-levels

Core-levels PrFeO3 Pr0.9Ca0.1FeO3 Pr0.8Ca0.2FeO3 Fe2O3 [19] CaO [18]

Pr 3d 20.3 20.5 21.3 — —

Fe 2p 14.0 13.8 13.6 13.2 —

Ca 2p — 4.3 4.8 — 3.6

S.K. Pandey et al. / Physica B 365 (2005) 47–5454

the exchange splitting increases by 0.5 eV. This isagain due to more pulling of the states nearer tonucleus than that farther from it.

4. Conclusion

In conclusion we have successfully used thecombustion method for synthesizing the com-pounds Pr1�xCaxFeO3 for x ¼ 0, 0.1 and 0.2.The XRD studies showed that the structureremained orthorhombic and distortion of FeO6

octahedra decreased on Ca doping. Fe K-edgeabsorption studies showed that the ionic state ofFe was 3+ and its effective charge increased onhole doping. XPS studies of Pr 3d, Fe 2p, Ca 2pand O 1s showed that the ionic state of Pr, Fe andCa was close to the ionic state of these elements inPr2O3, Fe2O3 and CaO, respectively. On Cadoping effective charge on Fe and Pr increasedwhereas that on O decreased. Change in exchangesplitting consistent with the increased effectivecharge was also observed on Ca doping. Work onother compounds and the detailed XPS andXANES studies on the above compounds are inprogress.

We would like to acknowledge R. K. Sahu,P. Saravanan and V. G. Sathe and N. P. Lalla and

S. Bhardwaj for help with synthesis, XAS andXRD, respectively. SKP is thankful to UGC-DAECSR for financial support.

References

[1] M. Imada, et al., Rev. Mod. Phys. 70 (1998) 1039.

[2] S. Kawasaki, et al., J. Phys. Soc. Jpn. 67 (1998) 1529.

[3] P.M. Woodward, et al., Phys. Rev. B 62 (2000) 844.

[4] M. Takano, Mater. Res. Bull. 12 (1977) 923.

[5] T. Akao, et al., Phys. Rev. Lett. 91 (2003) 156405.

[6] M. Abbate, et al., Phys. Rev. B 46 (1992) 4511.

[7] A.E. Bocquet, et al., Phys. Rev. B 45 (1992) 1561.

[8] C.C. Hwang, et al., Mater. Sci. Eng. B 111 (2004)

49.

[9] R.A. Young, et al., J. Appl. Crystallogr. 21 (1988)

416.

[10] S.K. Deshpande, et al., Pramana-J. Phys. 37 (1991)

373.

[11] T.M. Rearick, et al., Phys. Rev. B 48 (1993) 224.

[12] M. Marezio, et al., Acta Crys. B 26 (1970) 2008.

[13] W. Kraus, et al., J. Appl. Crys. 29 (1996) 301.

[14] J.M.D. Coey, et al., Adv. Phys. 48 (1999) 167.

[15] L.A. Grunes, Phys. Rev. B 27 (1983) 2111.

[16] A. Chainani, et al., Phys. Rev. B 48 (1993) 14818.

[17] D.D. Sarma, et al., J. Electron Spectrosc. Relat. Phenom.

20 (1980) 25.

[18] P. Selvam, et al., J. Electron Spectrosc. Relat. Phenom. 49

(1989) 203.

[19] B.J. Tan, et al., Chem. Mater. 2 (1990) 186.