Structural changes in gas hydrates and existence of a filled ice structure of methane hydrate above 40 GPa

Abstract

Recent studies have contributed a great deal to our understanding of the structural changes in gas hydrates under pressure. A feature of structural changes in terms of pressure and guest size is summarised in the former part of this paper. The gas hydrate with the guest size from argon to methane finally take a common filled ice I h structure, although they have different initial and intermediate structures. In the latter part, retention of the filled ice I h structure of methane hydrate up to 42 GPa is described. In situ X-ray diffractometry and optical observation revealed the existence of the filled ice structure with the volume change of 40% and large anisotropic compressibility. The distances between oxygen atoms estimated from the unit cell parameters suggested that symmetrization of hydrogen bonds occurred in the structure.