College Park, Maryland      June 6 - 10 , 2004

M2-D5 (12:00): Crystal structure of hydrogen clathrate hydrate by in-situ high-pressure neutron diffraction

K. A. Lokshin, Y. Zhao, D. He (Los Alamos Neutron Science Center, Los Alamos National Laboratory), W. L. Mao (Department of the Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA; Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA), H.-K. Mao, R. J. Hemley (Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA), M. V. Lobanov, M. Greenblatt (Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, NJ 08854, USA)

New experimental setup for neutron diffraction at high hydrostatic pressure (up to 10 kbar) and in the 10-300 K temperature range was developed at LANSCE, LANL. The detailed crystal structure information was determined for hydrogen clathrate hydrate as a function of temperature and pressure for the first time. We found that hydrogen occupancy in the (32+x)H2*136H2O, x = 0-12 clathrate can be reversibly varied by changing the large (hexakaidecahedral) cage occupancy between 2 and 4 molecules, but keeping single occupancy of the small (dodecahedral) cage. Above 130-160 K the guest hydrogen molecules were found in the delocalized state, rotating around the centres of the cages. Decrease of temperature results in the rotation freezing followed by a complete localization below 50 K.

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