Elastic properties of sII clathrate hydrates and its dependence on the size of guest molecule

本研究的目的是确定具有II型结构的单晶气体笼形水合物弹性性能的压力依赖性，并阐明其机械稳定性和主-客气相互作用。结果表明：利用新型氦气驱动金刚石砧细胞生长出了水合氮、水合氩、水合氪和四氢呋喃的单晶，并通过显微镜和高压拉曼光谱目视观察了它们的压力诱导相变。结果表明，具有立方结构II的气体水合物最终通过H结构转化为无笼的“充冰”，且氮水合物、氩水合物和氪水合物的立方结构II的压力区分别为0.1 ~ 0.85 GPa、0.2 ~ 0.43 GPa和0.01 ~ 0.45 GPa。,分别。此外，我们还对立方结构II相的水合氩和水合氪单晶进行了高压布里渊分析，以确定它们的弹性性能与压力的关系。通过分析，我们发现II型结构天然气水合物的弹性性质几乎是弹性各向同性的，并且与立方结构I型甲烷水合物的弹性性质相似。然而，II型结构天然气水合物的弹性常数、压力依赖性和压力区取决于客体分子（原子）的大小。因此，主水-客体气相互作用和机械稳定性与客体分子相对于主水笼的大小有关。

The purpose of this research is to determine the pressure dependences of elastic properties of single crystalline gas clathrate hydrates with structure II, and to clarify the mechanical stability and the host water-guest gas interaction. The results are as follows: We grew the single crystals of nitrogen hydrate, argon hydrate, krypton hydrate, and THF hydrate by using a new helium gas driven diamond anvil cell, and investigated their pressure-induced phase transformations by the visual observation through the microscope and the high-pressure Raman spectroscopy. From these results, it is found that the gas hydrates with cubic structure II are eventually transformed to the cage-less "filled ice" by way of structure H, and the pressure regions of cubic structure II for nitrogen hydrate, argon hydrate, and krypton hydrate are 0.1-0.85 GPa, 0.2-0.43 GPa, and 0.01-0.45 GPa., respectively. Moreover, we applied the high-pressure Brillouin analysis to the single crystals of argon hydrate and krypton hydrate in cubic structure II phase in order to determine their pressure dependences of elastic properties. From this analysis, we found that the elastic properties of the structure II gas hydrates are almost elastically isotropic and are similar to that of the cubic structure I methane hydrate. However, the elastic constants, the pressure dependences, and the pressure regions of structure II gas hydrates are dependent on the size of guest molecules (atoms). Therefore, the host water-guest gas interaction and the mechanical stability have relation to the size of guest molecule relative to that of host water cage.