High-pressure structures and vibrational spectra of barium fluoride: Results obtained under nearly hydrostatic conditions

摘要

Powder x-ray diffraction and Raman spectroscopy experiments, together with first-principles calculations, have been carried out to corroborate and clarify the pressure-dependent structural evolution of barium fluoride. X-ray diffraction experiments were performed both with and without a pressure-transmitting medium. The latter serve to reproduce past experimental results, while the former show a marked difference. Specifically, the experiments employing helium as a pressure-transmitting medium yield observed relative intensities and volume compression consistent with the proposed Ni_2In structure of barium fluoride at pressures greater than 14 GPa. The Raman spectroscopy measurements corroborate the proposed Ni_2In structure, as the spectra exhibit the two modes expected for the high-pressure phase. The experiments also demonstrate that barium fluoride remains an insulator up to at least 77 GPa, and the results of first-principles calculations suggest that a subsequent insulator-metal transition may not be expected well into the megabar regime.