First-principle calculations of anomalous spin-state excitation in LaCoO3

We investigate the different spin states of LaCoO3 employing the state-of-the-art ab initio band structure calculations within a rotationally invariant formulation of local density approximation (LDA) + U approach. The various magnetically ordered spin states of different supercells have been studied, including the low-spin state (LS), intermediate-spin state (IS), high-spin state (HS) Co3+ ions, as well as all combinations among these three states. The ground state is correctly predicted to be an insulator nonmagnetic state. Our calculations, together with previous susceptibility measurements for IS excitations in the LS ground state, lead to the conclusion that the nonmagnetic–paramagnetic transition in LaCoO3 at 90 K is caused by a gradual population of IS Co3+ ionic states. Our results show that the first thermally excited spin-state occurs from LS to an LS (CoLS3+ = 87.5%)–IS (CoIS3+ = 12.5%) ordered state, which can be distinguished from the LS–HS or IS state. We find that the mixture of LS–IS, LS–HS, and HS–IS spin states may develop an orbital ordering.