The vibrational properties of xenon hydrate: An inelastic incoherent neutron scattering study

We report results from a high-resolution, incoherent inelastic neutron scattering (IINS) study of xenon hydrate. This study extends previous work in which the existence of a strong coupling between localized guest vibrations and the lattice modes was shown(1) for the first time [Tse , Europhys. Lett., 54, 354 (2001)]. This guest-host coupling might be responsible for the glass-like temperature dependence of the thermal conductivity of the crystalline gas hydrates. Our experiment focused on the low-frequency phonon density of states of the ice-like water lattice of xenon hydrate. We found two broad maxima in the density of states (DOS) at energy transfers of 7.3 and 10.3 meV. The first peak is assigned to the transverse acoustic (TA) phonons near the zone boundary and the second to the fold-back of the TA modes towards the zone center. The guest-host coupling could be confirmed by finding three distinct low energy peaks in the DOS at energy transfers of 2.05, 2.87, and 3.94 meV. In addition, another broad inelastic scattering component extending from 0 to 1.5 meV has been found, which may also be important for the low-temperature behavior of the thermal conductivity. The line positions of the coupled modes shift to higher frequencies with increasing temperature, pointing towards the importance of the repulsive part of the host-guest interaction which is responsible for the stability of gas hydrate structures. (C) 2002 American Institute of Physics.