Pressure-Induced Intermetallic Valence Transition in BiNiO$_{3}$

The valence state change of BiNiO$_{3}$ perovskite under pressure has been investigated by a powder neutron diffraction study and electronic state calculations. At ambient pressure, BiNiO$_{3}$ has the unusual charge distribution Bi$^{3+}_{0.5}$Bi$^{5+}_{0.5}$Ni$^{2+}$O$_{3}$ with ordering of Bi$^{3+}$ and Bi$^{5+}$ charges on the A sites of a highly distorted perovskite structure. High pressure neutron diffraction measurements and Bond valence sum calculations show that the pressure-induced melting of the charge disproportionated state leads to a simultaneous charge transfer from Ni to Bi, so that the high pressure phase is metallic Bi$^{3+}$Ni$^{3+}$O$_{3}$. This unprecedented charge transfer between A and B site cations coupled to electronic instabilities at both sites leads to a variety of ground states, and it is predicted that a Ni-charge disproportionated state should also be observable. \newline [1] M. Azuma \textit{et al}.\textit{, J. Am. Chem. Soc.}, \textbf{129}, (2007) 14433.