Low-temperature and high-pressure experiments were performed with filled ice structures of methane hydrate and hydrogen hydrate under pressure and temperature conditions of 2.0-77 GPa and 30-300 K and 5.0-50 GPa and 10-300 K, respectively, using diamond anvil cell and helium-refrigerator cryostat. In-situ X-ray diffractometry and Raman spectroscopy revealed that orientational ordering of guest molecules occurred from disordered-rotation state at low temperature and high pressure, and that the guest ordering induced structural changes. For hydrogen hydrate, cubic structure was deformed to a tetragonal structure, while for methane hydrate axial ratios of orthorhombic structure were remarkably changed within the same orthorhombic structure.
