60 巻 (2018) 1 号 p. 54-61
Gas hydrates, clathrate compounds, consist of hydrogen-bonded water molecules forming cages and of guest species included in the host cages. A wide variety of gas hydrate exhibits phase changes depending on pressure and guest size, and finally transforms to filled ice structures. Upon phase transition between cage structures, e.g. sⅠ to sH of methane hydrate, a characteristic “cage recombination mechanism” using an analogy from genetic recombination was observed, while upon sH to filled ice structure, typical reconstructive mechanism was observed. At low temperatures and high pressures phase changes induced orientational ordering of guest molecules occurs for methane hydrate and hydrogen hydrate. At high temperatures and high pressures, methane hydrate decomposes to solid methane and ice Ⅶ. The stability region and decomposition curve was obtained.