Direct observation of the pressure-induced charge redistribution inBiNiO3by x-ray absorption spectroscopy

Abstract

To investigate the change in the electronic structure of ${\text{BiNiO}}_{3}$ accompanied by metal-insulator (MI) transition, we measured x-ray absorption spectroscopy (XAS) spectra at the $\text{Ni}\text{ }K$ and $\text{Bi}\text{ }L$ edges under various pressures up to 6 GPa. Both $\text{Bi}\text{ }{L}_{3}$ and $\text{Ni}\text{ }K$ edge XAS spectra clearly change at 4 GPa, indicating the electronic state in Bi and Ni ion changes. A quantitative analysis of the $\text{Ni}\text{ }K$ edge spectra in the pre-edge region based on the charge-transfer cluster model including multiplet terms revealed that the electronic configuration changes from ${d}^{8}$ in the insulating phase to $56%\text{ }{d}^{7}+44%\text{ }{d}^{8}\underset{̱}{L}$ in the metal phase. From these results, we concluded that the MI transition in ${\text{BiNiO}}_{3}$ is induced by the collapse of charge-transfer gap and is governed by the redistribution of $\text{O}\text{ }2p$ ligand holes.