Structural changes and hydration-number changes of gas hydrate under high pressure

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Three high-pressure structures of methane hydrate, a hexagonal structure (str.A) and two orthorhombic structures (str.B and str.C), were found by in-situ x-ray diffractometry and Raman spectroscopy. The well-known structure I (str.I) decomposed into the str.A and fluid at 0.8 GPa. The str.A transformed into the str.B at 1.6 GPa, and the str.B further transformed into the str.C at 2.1 GPa which survived above 7.8 GPa. The fluid solidified as ice VI at 1.4 GPa, and the ice VI transformed to ice VII at 2.1 GPa. The structural changes occurring with increasing pressure were observed reversibly with decreasing pressure.The structural analysis revealed that the str.A was a primitive hexagonal lattice with the unit cell parameters of a=1.2004 nm and c=1.0046 nm at 0.8 GPa. The framework of the hexagonal structure is composed of three different cages, one 20-hedra cage, three 12-hedra cages and two modified 12-hedra cages within a unit cell. In order to stabilize the structure only by small methane molecules, it is expected that two methane molecules are accommodated in the 20-hedra. As for two orthorhombic structures, the str.B and str.C, the unit cell parameters tentatively determined were a=0.8140, b=0.7935 nm, c=0.4806 nm at 1.8 GPa and a=0.7572 nm, b=0.7967 nm, c=0.4644 nm at 4.80 GPa, respectively. The str.C survived above 7.8 GPa. The bulk modulus is comparable to that of ice VII. These suggest that the str.C is probably a denser structure rather than other structures consisting of cages.