Theoretical investigation of the electronic structure of LaCoO3 by ab initio molecular-orbital calculations.

Abstract

Electronic structures of LaCo${\mathrm{O}}_{3}$ 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 LaCo${\mathrm{O}}_{3}$ containing the ${[\mathrm{C}\mathrm{o}\ensuremath{-}{\mathrm{O}}_{6}]}^{9\ensuremath{-}}$ cluster. The obtained results are summarized as follows: (1) At low temperatures, LaCo${\mathrm{O}}_{3}$ 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 photo-electron spectroscopy and with the electronic structures derived from the charge-transfer model proposed by Sawatzky and Allen. (2) The magnitudes of the band gap ${\ensuremath{\Delta}}^{\ensuremath{'}}$, the charge-transfer energy $\ensuremath{\Delta}$, and the $d\ensuremath{-}d$ Coulomb interaction energy $U$ in the high-spin state are larger than those in the low-spin state. (3) The covalency of LaCo${\mathrm{O}}_{3}$ 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), there is a positive correlation between the magnitudes of ${\ensuremath{\Delta}}^{\ensuremath{'}}$, $\ensuremath{\Delta}$, and $U$ and the ionicity of LaCo${\mathrm{O}}_{3}$. (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.