High-pressure structures of methane hydrate observed up to 8 GPa at room temperature

Three high-pressure structures of methane hydrate, a hexagonal structure (s tr.A) and two orthorhombic structures (str.B and str.C), were found by in s itu 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 transfor med into the str.B at 1.6 GPa, and the str.B further transformed into the s tr.C at 2.1 GPa which survived above 7.8 GPa. The fluid solidified as ice V I at 1.4 GPa, and the ice VI transformed to ice VII at 2.1 GPa. The structu ral changes occurring with increasing pressure were observed reversibly wit h decreasing pressure. The symmetric stretching vibration, nu (1), of the m ethane molecule observed in the Raman spectra changed along with the struct ural changes. The bulk moduli, K-0, for the str.I, str.A, and str.C were ca lculated to be 7.4, 9.8, and 25.0 GPa, respectively. The difference in the bulk moduli implies the difference in fundamental structure of the high-pre ssure structures. (C) 2001 American Institute of Physics.