Structures and Dynamics of Amorphous Clathrate Hydrates

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At beginning of the present project, we have developed a vapor-deposition line for preparation of the amorphous samples at low temperature. We have also made major modification on a cryostat for in situ neutron scattering measurements of vapor-deposited samples to use it on MARI spectrometer of Rutherford-Appleton Laboratory.We have prepared amorphous clathrate hydrates of Xe, SF_6, CD_4, Ar gases by using the above apparatus and performed in situ neutron scattering measurements in a temperature range 10-200K. The used neutron spectrometers are HIT (neutron diffraction), SWAN (small-angle neutron scattering), and LAM-D (incoherent inelastic neutron scattering) of High Energy Accelerator Research Organization (KEK), and MARI (high-intensity coherent inelastic neutron scattering) in Rutherford-Appleton Laboratory (RAL, UK).Neutron diffraction and small-angle scattering data revealed that increasing guest content and/or guest size facilitates formation of hydrogen-bonds and resultant local cage like structure of water molecules. These are due to hydroforbic hydration effect. Furthermore, amorphous clathrate hydrates transformed to clathrate hydrate crystals (not ice and guest crystals) on heating the samples up to around 150 K. Incoherent inelastic scattering revealed that above hydrogen-bond formation decreases the intensity of the low-energy phonon excitations. Coherent inelastic scattering showed the quasi-dispersive phonon excitation in amorphous ice and amorphous Xe clathrate hydrate, indicating the existence of quasi-Brillouin zone in amorphous hydrogen-bonded systems.Thus, we have prepared new materials, amorphous clathrate hydrates, and investigated their structures and dynamics systematically by using various neutron scattering methods.