A possible existence of phase change of deuterated ice VII at about 11 GPa by X-ray and Raman studies

High pressure experiments were performed with deuterated ice VII using diamond anvil cell in a pressure range of 0.1 MPa to 60 GPa at room temperature in order to further understanding a long-argued issue of phase change in ice VII at approximately 11 GPa. In-situ X-ray diffractometry revealed splitting of diffraction lines of cubic ice VII above 11GPa, which were indexed as a tetragonal structure. The tetragonal structure survived at least up to 60 GPa. Pressure versus volume curves of cubic and tetragonal structures were obtained and the bulk moduli were calculated. Raman spectroscopy showed that the full width at half maximum of O-D vibrational mode decreased with increasing pressure, and showed minimum value at 11 GPa, and then it became broader again above this pressure. The squared vibrational frequency changed linearly with pressure, and the slope changed at about 14 GPa, indicating existence of phase change. All experimental results evidently supported the existence of phase change of ice VII at approximately 11 GPa.