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;he surrounding hydrogen-bonde d network. Rietveld analysis revealed that the magnitude of the thermal par ameters of the host atoms of the CO2 clathrate hydrate depends strongly on the crystallographic site, while those of the Xe clathrate hydrate do not d epend on the site. The host lattice structure of the Xe clathrate hydrate m ay be a close representation of the hypothetical empty clathrate hydrate, b ecause of the simple spherical shape of the Xe molecule. The site dependenc e 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 mol ecule rotates anisotropically about the symmetry axis inside;the tetrakaide cahedral cage. We suppose that the surrounding deuterium atoms are temporar ily attracted and released by the oxygen atoms of the rotating CO2 molecule and so their thermal parameters are large. It is concluded that the intera ction between the guest molecule and the surrounding host atoms is so stron g that the thermal vibrations of the host atoms are affected by the motion of the guest molecules inside the cages.