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

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.