Direct observation of the pressure-induced charge redistribution in BiNiO3 by x-ray absorption spectroscopy

To investigate the change in the electronic structure of BiNiO3 accompanied by metal-insulator (MI) transition, we measured x-ray absorption spectroscopy (XAS) spectra at the Ni K and Bi L edges under various pressures up to 6 GPa. Both Bi L-3 and Ni K edge XAS spectra clearly change at 4 GPa, indicating the electronic state in Bi and Ni ion changes. A quantitative analysis of the Ni 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% d(7)+44% d(8)L_ in the metal phase. From these results, we concluded that the MI transition in BiNiO3 is induced by the collapse of charge-transfer gap and is governed by the redistribution of O 2p ligand holes.