Spin-state transition and spin-polaron physics in cobalt oxide perovskites: ab initio approach based on quantum chemical methods

A fully ab initio scheme based on quantum chemical wavefunction methods is used to investigate the correlated multiorbital electronic structure of a 3d-metal compound, LaCoO₃. The strong short-range electron correlations, involving both Co and O orbitals, are treated by multireference techniques. The use of effective parameters like the Hubbard U and interorbital U′, J terms and the problems associated with their explicit calculation are avoided with this approach. We provide new insight into the spin-state transition at about 90 K and the nature of charge carriers in the doped material. Our results indicate the formation of a t_2g ⁴e_g ² high-spin state in LaCoO₃ for T > 90K. Additionally, we explain the paramagnetic phase in the low-temperature lightly doped compound through the formation of Zhang-Rice-like O hole states and ferromagnetic clusters.