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It is well known that at low temperatures the molecular crystal SF6 exists in an orientationally disordered body-centered cubic phase (from 90 – 230 K) and an ordered monoclinic phase (below 90 K). The high-pressure behaviour has so far been only investigated by Raman spectroscopy with pressures up to 10 GPa. Sasaki et al. propose a phase transtition at 0.25 GPa from liquid SF6 to the so-called solid I phase and a second phase transition at 1.8 GPa to the solid II phase. Moreover the assumption is made that the solid I phase crystallizes in the bcc and the solid II phase in the monoclinic structure. This study presents an in-situ crystal-structure determination of SF6 in diamond anvil cells (DAC) up to 14 GPa. In order to ensure quasi-hydrostatic conditions, a mixture of 20 vol% SF6 in helium was loaded into the DAC. During compression of the gas mixture, SF6 separated from the He at around 0.5 GPa. Crystal growth was observed at 2 GPa and after a pressure increase to 4 GPa SF6 single crystals were grown using an external heating set-up. In-situ Raman measurements show the typical SF6 modes and indicate a phase transition between 1.6 and 2.2 GPa consistent with earlier results. X-ray diffraction experiments have been performed at the Extreme Conditions Beamline P02.2 at PETRA III. Single-crystal and powder data were measured using 43 keV radiation at four different pressure points: 1.5 GPa, 1.9 GPa, 4 GPa and 14 GPa. At 1.5 and 1.9 GPa the structure was determined to be the proposed orientationally disordered body-centered cubic and at 4 GPa SF6 crystallizes in the monoclinic structure. After further compression to 14 GPa only powder rings were observed and the monoclinic phase is still stable.

High-pressure X-ray single-crystal and powder diffraction of SF6up to 14 GPa

High-pressure X-ray single-crystal and powder diffraction of SF₆up to 14 GPa