Pressure-induced phase changes of argon hydrate and methane hydrate at room temperature

High-pressure experiments of argon hydrate and methane hydrate were performed using a diamond anvil cell in a pressure range of 0.2 to 10.0 GPa at room temperature. In-situ X-ray diffractometry and optical microscopy revealed that two high pressure structures of argon hydrate, a primitive tetragonal structure and a body-centered orthorhombic structure, existed under pressures of up to 6.5 GPa. The structural analysis showed that the tetragonal structure was composed solely of 14-hedra accommodating two argon atoms, and that the body-centered orthorhombic structure belonged to a filled-ice structure, i.e., a new type of structure in a water-guest system. As for methane hydrate, three high-pressure structures, a hexagonal structure, a primitive orthorhombic structure, and a body-centered orthorhombic structure, were found. The structural analysis indicated that the hexagonal structure was a modified structure of a hexagonal one reported at ambient pressure, and that the latter orthorhombic structure was the filled-ice structure.