Phase Transition of a Structure II Cubic Clathrate Hydrate to a Tetragonal Form

The crystal structure and phase transition of cubic structure II (sII) binary clathrate hydrates of methane (CH₄) and propanol are reported from powder X-ray diffraction measurements. The deformation of host water cages at the cubic–tetragonal phase transition of 2-propanol+CH₄hydrate, but not 1-propanol+CH₄hydrate, was observed below about 110 K. It is shown that the deformation of the host water cages of 2-propanol+CH₄hydrate can be explained by the restriction of the motion of 2-propanol within the 5¹²6⁴host water cages. This result provides a low-temperature structure due to a temperature-induced symmetry-lowering transition of clathrate hydrate. This is the first example of a cubic structure of the common clathrate hydrate families at a fixed composition.