ELECTRONIC-STRUCTURE AND SPIN-STATE TRANSITION OF LACOO3

We present soft-x-ray absorption spectra (XAS) of LaCoO3 taken at diff erent temperatures (80-630 K). The shape of the multiplets in the Co 2 p XAS spectra conveys information on the symmetry and spin of the grou nd state. The 0 Is XAS spectra are related to unoccupied metal bands t hrough covalent mixing. The changes in the spectra taken at different temperatures provide information on the spin-state transition in this compound. At low temperature, 80 and 300 K, the material is in a highl y covalent low-spin state. The main contribution to the ground state i n this case is given by t2g6 (1A1) with an occupancy of 0.56. At highe r temperature, 550 and 630 K, the results indicate a gradual transitio n to a mixed-spin state. The main contribution to the high-spin part o f the mixture is given by t2g4e(g)2(5T2) with an occupancy of 0.71. Th ere is no evidence of charge disproportionation in the temperature ran ge 80-630 K. The O 1s XAS spectra reflect important changes in the uno ccupied Co 3d bands across the spin-state transition.