Structural changes of filled ice Ic structure for hydrogen hydrate under high pressure

High-pressure experiments of hydrogen hydrate, filled ice Ic structure, were performed using a diamond-anvil cell in the pressure range of 0.1-80.3 GPa at room temperature. In situ x-ray diffractometry (XRD) revealed that structural changes took place at approximately 35-40 and 55-60 GPa, and that the high-pressure phase of hydrogen hydrate survived up to at least 80.3 GPa. Raman spectroscopy showed that the changes in vibrational mode for the hydrogen molecules in hydrogen hydrate occurred at around 40 and 60 GPa, and these results were consistent with those of the XRD. At about 40 GPa, the intermolecular distance of host water molecules consisting the framework attained the critical distance of symmetrization of the hydrogen bond for water molecules, which suggested that symmetrization of the hydrogen bond occurred at around 40 GPa. The symmetrization might introduce some structural change in the filled ice Ic structure. In addition, the existence of the high-pressure phase above 55-60 GPa implies that a denser structure than that of filled ice Ic may exist in hydrogen hydrate.