A high-pressure phase of Na2CuF4 with eight-coordinated Cu2+ – a low-pressure analogue of Mg2SiO4

Abstract

Density functional theory calculations and Raman spectroscopy measurements indicate that compression of Na₂CuF₄ results in a series of phase transitions leading through distorted variants of the Sr₂PbO₄ (post-perovskite) structure up to a Th₃P₄-like polymorph featuring eightfold-coordinated Cu²⁺ cations. The properties of this phase of Na₂CuF₄ at pressures below 100 GPa closely mimic those of an analogous structure of Mg₂SiO₄ which is predicted to form in the mantle of rocky planets under sub-TPa conditions. The possible synthesis of a Na₂CuF₄ phase resembling the Th₃P₄-type structure at room temperature and relatively low pressures opens new avenues for exploring the geochemistry of super-Earth mantles, as well as obtaining Th₃P₄-type structures in fluoride systems of applicational importance.