Distortion of Host Lattice in Clathrate Hydrate as a Function of Guest Molecule and Temperature

Neutron powder diffraction data of deuterated clathrate hydrates of carbon dioxide and xenon were measured in order to investigate the effects of the motion of guest molecules on the structure of the surrounding hydrogen-bonded network. Rietveld analysis revealed that the magnitude of the thermal parameters of the host atoms of the CO2 clathrate hydrate depends strongly on the crystallographic site, while those of the Xe clathrate hydrate do not depend on the site. The host lattice structure of the Xe clathrate hydrate may be a close representation of the hypothetical empty clathrate hydrate, because of the simple spherical shape of the Xe molecule. The site dependence of the thermal parameters of the host atoms in the CO2 clathrate hydrate are attributed to the motion of the CO2 molecules in the cages. The CO2 molecule rotates anisotropically about the symmetry axis inside the tetrakaidecahedral cage. We suppose that the surrounding deuterium atoms are temporarily attracted and released by the oxygen ato...