High-pressure transition to the post-barite phase in BaCr04 hashemite.

A recent high-pressure study on barium Chromate BaCr04 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 Aß04-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 P212121 post-barite-type phase, recently reported for BaS04. 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 Cr04 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. [ABSTRACT FROM AUTHOR]