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