High pressure X-ray diffraction and Raman spectroscopic studies of the phase change of D₂O ice VII at approximately 11 GPa

Abstract

High pressure experiments were performed on D₂O ice VII using a diamond anvil cell in a pressure range of 2.0–60 GPa at room temperature. In situ X-ray diffractometry revealed that the structure changed from cubic to a low symmetry phase at approximately 11 GPa, based on the observed splitting of the cubic structure's diffraction lines. Heating treatments were added for the samples to reduce the effect of non-hydrostatic stress. After heating, splitting diffraction lines became sharp and the splitting was clearly retained. Although symmetry and structure of the transformed phase have not been determined, change in volumes vs. pressure was calculated, assuming that the low-symmetry phase had a tetragonal structure. The bulk modulus calculated for the low-symmetry phase was slightly larger than that for the cubic structure. In Raman spectroscopy, the squared vibrational frequencies of ν₁ (A_1g), as a function of pressure, showed a clear change in the slope at 11–13 GPa. The full width at half maxima of the O-D modes decreased with increasing pressure, reaching a minimum at approximately 11 GPa, and increased again above 11 GPa. These results evidently support the existence of phase change at approximately 11 GPa for D₂O ice VII.

Keywords:

• phase transition
• D₂O ice VII
• high pressure
• X-ray diffraction
• Raman spectroscopy

Acknowledgements

We thank Dr H. Hirao, the beamline scientist of BL-10XU at SPring8, for assistance with XRD experiments. This research was supported partly by Grant-in-Aid for Scientific Research (A) (Grant 22244055) from Japan Society for the Promotion of Science.