H. C. N. Tolentino

Bandwidth-driven nature of the pressure-induced metal state of LaMnO3

Using X-ray absorption spectroscopy (XAS), we studied the local structure in LaMnO3 under applied pressure across and well above the insulator-to-metal (IM) transition. A hysteretic behavior points to the coexistence of two phases within a large pressure range (7 to 25 GPa). The ambient phase with highly Jahn-Teller (JT) distorted MnO6 octahedra is progressively substituted by a new phase with less-distorted JT MnO6 units. The electronic delocalization leading to the IM transition is finger-printed from the pre-edge XAS structure around 30 GPa. We observed that the phase transition takes place without any significant reduction of the JT distortion. This entails band overlap as the driving mechanism of the IM transition.

Temperature and high-pressure dependent x-ray absorption ofSmNiO3 at the Ni K and Sm L3 edges

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.

Stability of the Ni sites across the pressure-induced metallization in YNiO3

The local environment of nickel atoms in Y NiO3 across the pressure- induced insulator to metal (IM) transition was studied using X-ray absorption spectroscopy (XAS) supported by ab initio calculations. The monotonic contraction of the NiO6 units under applied pressure observed up to 13 GPa, stops in a limited pressure domain around 14 GPa, before resuming above 16 GPa. In this narrow pressure range, crystallographic modifications basically occur in the medium/long range, not in the NiO6 octahedron, whereas the evolution of the near-edge XAS features can be associated to metallization. Ab initio calculations show that these features are related to medium range order, provided that the Ni-O-Ni angle enables a proper overlap of the Ni eg and O 2p orbitals. Metallization is then not directly related to modifications in the average local geometry of the NiO6 units but more likely to an inter-octahedra rearrangement. These outcomes provides evidences of the bandwidth driven nature of the IM transition.