Coupling of localized guest vibrations with the lattice modes in clathratehydrates

The thermal conductivity of a gas hydrate is much lower than that of ice an d more intriguingly, in spite of the crystalline structure, the temperature profile resembles an amorphous solid. Here we present results from neutron incoherent inelastic scattering (IINS) experiments and theoretical relaxat ion time calculations to demonstrate unambiguously the coupling of guest an d water framework vibrations. The coupling is the consequence of symmetry f orbidden crossings of the localized guest motions with the acoustic branche s of the lattice vibrations. This phonon interaction leads to an energy exc hange mechanism that explains the anomalous thermal behaviour.