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

高压下天然气水合物的结构变化和水化数变化

基本信息

批准号：
12650915
负责人：
YAGI Hisako
金额：
$ 2.37万
依托单位：
University of Tsukuba
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2000
资助国家：
日本
起止时间：
2000 至 2001
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-12650915/
关键词：
gas hydrate methane hydrate high pressure structural change hydration number cage occupancy diamond anvil cell 高圧

项目摘要

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.

通过原位X射线衍射和拉曼光谱分析，发现甲烷水合物具有三种高压结构：六方结构（str.A）和正交结构（str.B和str.C）.在0.8GPa下，结构Ⅰ分解为结构A和流体。在1.6 GPa压力下，str.A转变为str.B，str.B在2.1 GPa压力下进一步转变为str.C，并在7.8 GPa压力以上存活。流体在1.4GPa下固化为冰VI，并且冰VI在2.1GPa下转化为冰VII。结构分析表明，str.A为原始六方晶格，在0.8GPa时晶胞参数为a= 1.2004nm，c= 1.0046nm.六边形结构的骨架由三个不同的笼组成，一个20面体笼，三个12面体笼和两个修改的12面体笼在一个晶胞内。为了仅通过小的甲烷分子来稳定结构，预期在20面体中容纳两个甲烷分子。对于两种正交结构，str.B和str. C，在1.8 GPa下的晶胞参数分别为a=0.8140，B=0.7935 nm，c=0.4806 nm;在4.80 GPa下的晶胞参数分别为a=0.7572 nm，B=0.7967 nm，c=0.4644 nm。str.C在7.8GPa以上存活。体积模量与冰VII的体积模量相当。这表明，str.C可能是一个密集的结构，而不是其他结构组成的笼。