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

Abstract

Wereport on x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine; structure (EXAFS) measurements of SmNiO3 from 20 Kto 600 Kand up to 38 GPa at the NiKand Sm; L3 edges. A multiple component pre-NiKedge tail is understood, originating from 1 s transitions to; 3d-4p states while a post-edge shoulder increases distinctively smoothly, at about the insulator to; metal phase transition (TIM), due to the reduction of electron–phonon interactions as the Ni 3d andO; 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; 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 Kthe 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 Kand 20 Kshow breaks in its monotonic; increase at the different structural changes. The SmL3 edge does not show distinctive behaviors either; at 300 Kor 20 Kup to about 35 GPa but the perovskite Smcage, coordinated to eight oxygen atoms,; undergoes strong uneven bond contractions at intermediate pressures where we found the coexistence; of octahedral and rhombohedral superexchange angle distortions.Wefound that the white line; pressure-dependent anomaly may be used as an accurate alternative for delineating pressure–; temperature phase diagrams.