High-pressure transition to the post-barite phase in BaCrO4hashemite

Abstract

A recent high-pressure study on barium chromate BaCrO${}_{4}$ reported a phase transition but the structure of the high-pressure phase structure could not be identified. This high-pressure phase was suggested to have a monoclinic structure different from other high-pressure forms of $AB$O${}_{4}$-type compounds. In this work, we have carried out x-ray diffraction measurements up to 46 GPa using He as the quasihydrostatic pressure medium and density-functional theory calculations. Our studies allow us to identify the high-pressure phase as the $P$2${}_{1}$2${}_{1}$2${}_{1}$ post-barite-type phase, recently reported for BaSO${}_{4}$. The equations of state of both, the low- and the high-pressure phases have been determined. Elastic constants of the initial barite-type phase were calculated and the shear and Young moduli, and the Poisson coefficient deduced from them. Finally Raman and infrared-active modes are also calculated, discussed, and compared with experiments when possible. As a first approximation, the vibrations can be separated in internal vibrations of the CrO${}_{4}$ tetrahedra and vibrations where the tetrahedral units behave as rigid units. Pressure coefficients of the different modes are reported being two soft modes found for the high-pressure phase.