Temperature and high-pressure dependent X-ray absorption of SmNiO3 at the Ni K- and Sm L3- edges

Abstract

We report on XANES and EXAFS measurements of SmNiO3 from 20 K to 600 K and up to 38 GPa at the Ni K- and Sm L3- edges. A multiple component pre- Ni K-edge tail is understood originating from 1s transitions to 3d-4p states while a post-edge shoulder increases distinctively smooth, at about the insulator to metal phase transition (TIM), due to the reduction of electron-phonon interactions as the Ni 3d and O 2p band overlap triggers the metallic phase. This effect is concomitant with pressure induced Ni-O-Ni angle increments toward more symmetric Ni3+ octahedra of the rhombohedral R-3c space group. Room temperature pressure dependent Ni white line peak energies have an abrupt ~3.10+-0.04 GPa Pa valence discontinuity from non-equivalent Ni3+δ + Ni3-δ charge disproportionate net unresolved absorber turning at ~TIM into Ni3+ of the orthorhombic Pbnm metal oxide phase. At 20 K the overall white line response, still distinctive at TIM ~8.1+-0.6 GPa is much smoother due to localization. Octahedral bond contraction up to 38 GPa and at 300 K and 20 K show breaks in its monotonic increase at the different structural changes. The Sm L3-edge does not show distinctive behaviors either at 300 K or 20 K up about 35 GPa but the perovskite Sm cage, coordinated to eight oxygen atoms, undergoes strong uneven bond contractions at intermediate pressures where we found coexistence of octahedral and rhombohedral superexchange angle distortions. We found that the white line pressure dependent anomaly may be used as an accurate alternative for delineating pressure-temperature phase diagrams.