Lattice dynamics of methane and xenon hydrate: Observation of symmetry-avoided crossing by experiment and theory

The collective dynamics of methane and xenon hydrate in the energy range of $\pm 20\mathrm{meV}$ and the Q range of $1.5–11{\mathrm{nm}}^{-1}$ has been investigated by inelastic x-ray scattering and compared to results from inelastic neutron scattering experiments and lattice dynamical calculations. The experiment focused on the low–frequency phonon dispersion curves that were found to point towards the existence of an avoided crossing between the acoustic lattice phonons and the localized guest modes. The calculations reproduce the experimental spectra and show that the localized vibrations of the guest molecules or atoms are mixed with the collective host lattice vibrations, leading to a damping of the intensity of the acoustic host lattice phonons. This observation supports the idea of a resonant scattering mechanism for the strong guest-host phonon interactions in clathrate hydrates.