  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+{\delta} +
Ni3-{\delta} 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.
