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 Hartree-Fock method, the unrestricted Hartree-Fock method, and the fourth-order Moller-Plesset perturbation method, and with basis sets appropriate for cobalt and oxygen atoms in order to reveal the electronic structures. 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, LaCoO3 is an insulator with the band structures, so that the upper Co 3d band, which contains a small contribution of the O 2p orbital, is above 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 experimental results obtained by photoelectron spectroscopy and with the electronic structures derived from the charge-transfer model proposed by Sawatzky and Alien. (2) The magnitudes of the band gap Delta', the charge-transfer energy Delta, and the d-d Coulomb interaction energy U in the high-spin state are larger than those in the low-spin state. (3) The 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 between Co and O orbitals. From (2) and (3), then is a positive correlation between the magnitudes of Delta', Delta, and U and the ionicity of LaCoO3. (4) The spin state transition occurs mainly due to the variation of Co-O bond length with increasing temperature. (5) The metal-insulator transition that appears at high temperatures is the charge-transfer-type transition.