Anharmonicity and guest-host coupling in clathrate hydrates

The dynamic response of Ar, Xe, O-2 and N-2-containing clathrate hydrates has been studied by neutron spectroscopy. Type I and type II clathrate hydrates show some small but significant differences in the low-frequency host contribution to the density of states. Both differ markedly from that of ice Ih and depend only weakly on the guest. While the vibrational modes associated with Xe and Ar are found to be close-to-harmonic, strong temperature dependent shifts are encountered for the O-2 and N-2 low-frequency vibrations. A coupling of the guest and host vibrations is confirmed by our experiments. Its strength depends strongly on the type of included guest. In particular Xe-hydrate cannot be considered representative as due to the large mass of the guest atom all its vibrational bands fall into the acoustic range of the host lattice.