Photon Factory Activity Report 2004 #22 Part B (2005)
Materials Science

10B, 9A/2004G086

X-ray absorption fine structure studies on La1-xSrxCoO3
U. Manju1, D. Matsumura2,3, T. Ohta2, D. D. Sarma*1
1
Solid state and Structural Chemistry Unit, Indian Institute of Science, Bangalore- 560 012, India.
2
Deptartment of Chemistry, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyoku, Tokyo 113.
3
Synchrotron Radiation Research Center, Japan Atomic Energy Institute, Mikazuki, Sayo-gun, Hyogo
679-5148, Japan.

Experiment
Polycrystalline samples of La1-xSrxCoO3 with x = 0, 0.05,0.1, 0.17, 0.2, 0.3 and 0.4 were prepared by solid statesynthesis using the starting materials La2O3, SrCO3 andCoC2O4. 2H2O. X-ray absorption measurements werecarried out at the beam line BL-10B and BL-9A to cover
the Co K, Sr K and La L3 absorption edges. Experiments
were performed at temperatures ranging from 25 K to300 K.
Results and discussion
Figure 1 shows the Co K-edge XANES spectra of La1SrxCoO3 collected at 25 K, exhibiting no drastic changexwith hole doping. Thus, the Co valency remains
essentially the same across the series. Thus this systembelongs to the charge-transfer insulator regime. The t2gand eg features and the 1s to 4p shake down processes are3+marked in the figure. These clearly show that Co in
6
LaCoO3, is not in a pure low spin (t2g ) state.

Conclusions
We probed the role of local distortions in bringing out
metal insulator transitions in La1-xSrxCoO3 system using xray absorption fine structure studies. Preliminary analysis
shows that there is an increase in the static disorder in theinsulating regime and after entering the metallic regime, it
saturates which possibly bring out a strong influence indriving MIT in the system. Further investigations andthorough analysis is required to come to a conclusion.

Absorption intensity (arb. unit)

Preliminary analysis of the Co K EXAFS region was
carried out using the nearest neighbour shell. It is seenthat the Co-O distance remains essentially the same
within a range of 0.005Å even after hole doping thesystem. Inset of figure 1 shows the relative mean squaredisplacement (RMSD) of bond lengths with respect totemperature for various compositions. In the figure,individual plots are shifted by 0.05 units for help of view.
We observed that the RMSD increases initially with theSr doping, but saturates after a critical composition.

T=25 K, Co- K edge

t2g

7710

1s

T=25 K
Co- K edge
La1-xSrxCoO3
with x=0-0.47715

Photon energy (eV)4p1sshake downeg

4p

1s

3d

x=0.4

RMSD (arb. unit)

Absorption intensity (arb. unit)

Introduction
Metal insulator transitions (MIT) in transition metaloxides is a widely studied problem in condensed matterphysics, but is still not completely understood. It is not
surprising that when we dope charge carriers into asystem, it undergoes a transition from an insulating to ametallic state, for example as exhibited by La1-xSrxCoO3[1] and La1-xSrxTiO3 [2]. But the extent of doping required
to bring about MIT is different for different systems. Also
it is surprising to note that certain family of compounds
like La1-xSrxFeO3 [3] and La1-xSrxCrO3 [4] remain insulating
for all values of x. To understand the role of local
distortions in bringing out such differences, we haveproposed to perform x-ray absorption fine structurestudies on some representative samples for different
concentrations and different temperatures. Here we havetaken LaCoO3 which is a nonmagnetic insulator. On
3+2+substitution of La by Sr , the system undergoes atransition to a metallic state at x~0.15 [1].

x=0.3
x=0.2
x=0.17x=0.1
x=0.05x=007700

7720

507740

Photon energy (eV)190

Users' Report

100

150
200
T (K)7760

2503007780

Fitting the data using correlated Einstein model, Einsteinfrequency and relative static mean square displacement
are extracted for each x value. It is observed that theEinstein frequency in the insulating region is higher thanthat in the metallic region. The static disorder increases
steadily with Sr concentration in the insulating regime
where as there is no change in the static disorder within
the metallic phase.

1.
2.
3.
4.

References
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J. Grenier, N. Ea, M. Pouchard and MM. AbouSekkina, Mat. Res. Bull. 19, 1301 (1984).
J. B. Webb, M. Sayer and A. Mansingh, Can. J.
Phys. 55, 1725 (1977).