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

用原位X射线衍射仪和拉曼光谱研究了甲烷水合物的三种高压结构，即六方结构(str.A)和两种正交结构(str.B和str.C)。熟知的结构I(strI.)在0.8 Gpa时分解为str.A和流体。A链在1.6 Gpa时转变为B链，B链在2.1 Gpa时进一步转化为C链，并在7.8 Gpa以上存活。流体在1.4 GPa时固化为冰VI，在2.1 GPa时由冰VI转变为冰VII。结构分析表明，该晶体为原始的六方晶格，晶胞参数为a=1.2004 nm，c=1.0046 nm。六角形结构的框架由三个不同的笼子组成，一个20面体的笼子，三个12面体的笼子和两个改进的12面体的笼子在一个单元内。为了仅通过甲烷小分子来稳定结构，预计在20面体中容纳两个甲烷分子。对于两种正交晶系结构str.B和str.C，初步确定的晶胞参数分别为a=0.8140，b=0.7935 nm，c=0.4806 nm和a=0.7572 nm，b=0.7967 nm，c=0.4644 nm。Str.C在7.8 Gpa以上存活下来。体积模数与冰的体积模数相当，这表明冰柱C可能是一种更致密的结构，而不是由笼子组成的其他结构。

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.