  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.
