Pressure-induced lntermetallic valence transition in BiNiO₃

The valence state change of BiNiO₃ perovskite under pressure has been investigated by a powder neutron diffraction study and electronic-state calculations. At ambient pressure, BiNiO₃ has the unusual charge distribution Bi³⁺_0.5Bi⁵⁺_0.5Ni ²⁺O₃ with ordering of Bi³⁺ 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 Bi³⁺Ni³⁺O₃. 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.