Coupling of localized guest vibrations with the lattice modes in clathrate hydrates

The thermal conductivity of a gas hydrate is much lower than that of ice and 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 relaxation time calculations to demonstrate unambiguously the coupling of guest and water framework vibrations. The coupling is the consequence of symmetry forbidden crossings of the localized guest motions with the acoustic branches of the lattice vibrations. This phonon interaction leads to an energy exchange mechanism that explains the anomalous thermal behaviour.