Pressure-induced charge amorphisation in BiNiO.

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 FeO, colossal magnetoresistances in manganites (e.g., LaCaMnO), and superconductivity in K-doped BaBiO. Here we report the observation of pressure induced charge amorphisation in a crystalline material. BiNiO has charge distribution BiBiNiO with long range order of the Bi and Bi 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. BiNiO provides fundamental insights to the study of amorphisation using charge states rather than atoms or molecules.