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 such as the Hubbard U and interorbital U', J terms and the problems associated with their explicit calculation are avoided with this approach. We compute the ordering of the lowest N-particle states in the parent compound and provide new insight into the nature of charge carriers in the hole-doped material. Our results suggest that the transition to a magnetically active state at about 90 K in LaCoO₃ involves a high-spin, t_2g⁴e_g² configuration. Additionally, we explain the paramagnetic phase in the low-temperature lightly doped compound through the formation of Zhang–Rice-like oxygen hole states and ferromagnetic clusters.