THEORETICAL INVESTIGATION OF THE ELECTRONIC-STRUCTURE OF LACOO3 BY AB-INITIO MOLECULAR-ORBITAL CALCULATIONS

Electronic structures of LaCoO3 near the Fermi level were investigated from ab initio molecular-orbital calculations using the restricted Ha rtree-Fock method, the unrestricted Hartree-Fock method, and the fourt h-order Moller-Plesset perturbation method, and with basis sets approp riate for cobalt and oxygen atoms in order to reveal the electronic st ructures. In the present analysis, we used the cluster composed of the finite unit cells of LaCoO3 containing the [Co-O-6](9-) cluster. The obtained results are summarized as follows: (1) At low temperatures, L aCoO3 is an insulator with the band structures, so that the upper Co 3 d band, which contains a small contribution of the O 2p orbital, is ab ove the Fermi level, and the O 2p and the lower Co 3d bands are below the Fermi level. These results are in good agreement with the experime ntal results obtained by photoelectron spectroscopy and with the elect ronic structures derived from the charge-transfer model proposed by Sa watzky and Alien. (2) The magnitudes of the band gap Delta', the charg e-transfer energy Delta, and the d-d Coulomb interaction energy U in t he high-spin state are larger than those in the low-spin state. (3) Th e covalency of LaCoO3 in the low-spin state is larger than that in the high-spin state due to the main contribution of the hybridization bet ween Co and O orbitals. From (2) and (3), then is a positive correlati on between the magnitudes of Delta', Delta, and U and the ionicity of LaCoO3. (4) The spin state transition occurs mainly due to the variati on of Co-O bond length with increasing temperature. (5) The metal-insu lator transition that appears at high temperatures is the charge-trans fer-type transition.