Intermediate-spin state and properties ofLaCoO3

The electronic structure of the perovskite ${\mathrm{LaCoO}}_3$ for different spin states of Co ions was calculated in the local-density approximation LDA+U approach. The ground state is found to be a nonmagnetic insulator with Co ions in a low-spin state. Somewhat higher in energy, we find two intermediate-spin states followed by a high-spin state at significantly higher energy. The calculations show that Co 3d states of ${\mathit{t}}_{2\mathit{g}}$ symmetry form narrow bands which could easily localize, while ${\mathit{e}}_{\mathit{g}}$ orbitals, due to their strong hybridization with the oxygen 2p states, form a broad σ* band. With temperature variation which is simulated by a corresponding change of the lattice parameters, a transition from the low- to intermediate-spin state occurs. This intermediate-spin (occupation ${\mathit{t}}_{2\mathit{g}}^5$ ${\mathit{e}}_{\mathit{g}}^1$) can develop an orbital ordering which can account for the nonmetallic nature of ${\mathrm{LaCoO}}_3$ at 90 K<T<500 K. Possible explanations of the magnetic behavior and gradual insulator-metal transition are suggested. © 1996 The American Physical Society.