Spin-orbit-induced mixed-spin ground state in RNiO3 perovskites probed by x-ray absorption spectroscopy : Insight into the metal-to-insulator transition

We report on a Ni L-2,L-3 edge x-ray absorption spectroscopy study in RNiO3 perovskites. These compounds exhibit a metal-to-insulator (MI) transition as temperature decreases. The L-3 edge presents a clear splitting in the insulating state, associated to a less hybridized ground state. Using charge transfer multiplet calculations, we establish the importance of the crystal field and 3d spin-orbit coupling to create a mixed-spin ground state. We explain the MI transition in RNiO3 perovskites in terms of modifications in the Ni3+ crystal field splitting that induces a spin transition from an essentially low-spin to a mixed-spin state.