Electronic structures of ${\mathrm{LaCoO}}_3$ and ${\mathrm{SrCoO}}_3$ cluster models near the Fermi level were investigated from ab initio molecular-orbital calculations using the Hartree-Fock approximation, the single-excitation configuration-interaction approximation, and local density approximations in order to reveal the electronic structures of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_3.$ The obtained results are summarized as follows: (1) ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_3$ is nearly intermediate between Mott-Hubbard-type compounds and charge-transfer-type compounds, with interaction between Co and O ions. (2) The covalency of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_3$ is due to the main contribution of the hybridization between Co and O orbitals. (3) With the decrease of the crystal field, i.e., the increase of Co-O bond length, the low-spin state gradually becomes unstable, while the high-spin state becomes stable. There is the possibility of the existence of an intermediate spin state. (4) The metal-insulator transition occurs due to the interaction between mixed states consisting of electrons for Co ions and holes for O ions. From the above results, the correlation between the spins of Co ions and O holes plays an important role in determining the physical properties of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_3.$

• Received 2 September 1997

DOI:https://doi.org/10.1103/PhysRevB.57.15211