Stability of Ni sites across the pressure-induced insulator-to-metal transition in YNiO 3

Abstract

The local environment of nickel atoms in YNiO 3 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 NiO 6 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 to long range, not in the NiO 6 octahedron, whereas the evolution of the near-edge XAS features can be associated with 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 e g and O 2p orbitals. Metallization is then not directly related to modifications in the average local geometry of the NiO 6 units but more likely to an interoctahedral rearrangement. These outcomes provide evidence of the bandwidth-driven nature of the IM transition.