W0092

Guest-Host Coupling in Gas Hydrates investigated with Inelastic Neutron Scattering. J. Baumert, C. Gutt, W. Press, IEAP Universität Kiel, Kiel, Germany, J.S. Tse, NRC, Ottawa, Canada.

One of the outstanding physical properties of gas hydrates is their thermal conductivity that is unusually low and displays a temperature dependence similar to that of glasses [1]. This is thought to arise from a strong coupling between guest molecule and host lattice vibrations, which we recently confirmed in a high-resolution inelastic neutron experiment (INS) on Xenon hydrate (Xe 5.75 H₂O) by finding three distinct low energy excitations at 2.05meV, 2.87meV and 3.94meV [2,3].

Xe-hydrate has the cubic structure I consisting of 6 large and 2 small cages in the unit cell. As the total incoherent scattering cross section of the water molecules in the unit cell exceeds the cross section of the Xe guest atoms by a factor of 1000, the INS signal arises mainly from the H-atoms and thus displays the density of states (DOS) of the host lattice vibrations. This way the Xe-hydrate is the model system to study the guest-host coupling in structure I gas hydrates.

Due to a symmetry avoided crossing [4] the acoustic phonons of the host lattice show a strong optic behavior in the frequency range of the localized modes of the Xe-atoms. As the Xe-atoms in the small spherical cage vibrate with the highest frequency and in the larger ellipsoidal cage with two lower frequencies, this gives rise to the three observed peaks.

The study of the guest-host coupling is now extended to methane hydrate and to Ar- and Kr-hydrates which have the cubic structure II. The combined results on these systems will allow us to study the influence of the guest mass and of the guest-host interaction strength on the coupling mechanism.

Reference:

[1] R.G.Ross, P.Andersson, G.Backstrom, Nature, 290, 322 (1981)

[2] J.S.Tse et al., accepted at Euro.Phys.Lett.

[3] C.Gutt, J.Baumert, W.Press, S.Janssen, J.S.Tse, to be published

[4] J.S.Tse, V.P.Shpakov, V.V.Murashov, V.R.Belosludov, J.Chem.Phys., 107, 9271 (1997)