Pressure-induced charge amorphisation in BiNiO₃

The order or disorder of electrons is fundamental to materials properties and also provides simple analogues to the different states of matter. A charge ordered (CO) insulating state, analogous to a crystalline solid, is observed in many mixed valence materials. On heating, this melts to a charge liquid (metallic) phase, often with interesting associated physics and functions such as the Verwey transition of Fe₃O₄, colossal magnetoresistances in manganites (e.g., La_0.5Ca_0.5MnO₃), and superconductivity in K-doped BaBiO₃. Here we report the observation of pressure induced charge amorphisation in a crystalline material. BiNiO₃ has charge distribution Bi³⁺_0.5Bi⁵⁺_0.5Ni²⁺O₃ with long range order of the Bi³⁺ and Bi⁵⁺ states at ambient pressure, but adopts another, structurally crystalline, but charge glassy, insulating phase at pressures of 4-5 GPa and temperatures below 200 K, before metallization above 6 GPa. This is analogous to the much-studied pressure induced amorphisations of many crystalline materials and melting is even observed at accessible pressure/temperature. BiNiO₃ provides fundamental insights to the study of amorphisation using charge states rather than atoms or molecules.