Lattice Dynamics Study Of Gas Hydrate

The well-known glass-like low thermal conductivity of gas hydrate and of semiconductor clathrate compounds with the same structure is now explained in consensus with the low energy guest-host coupling promoted by the resonance effect at the avoided crossings between localized rattling modes of the enclathrated guest species and the host lattice acoustic phonon branches of the same symmetry. This explanation is supported and supplemented by this thesis. This paper presents the algorithm for the calculation of the hydrogen positions in the ice lattice of gas hydrate with both structure I and structure II, which provides a complete coordinates of lattice structure for the computer simulation. Lattice dynamics simulations are then carried out for gas hydrates and inelastic neutron scattering experimental results are employed for comparison. The TIP4P oxygen-shell model and L-J potential used in our classical LD simulation are proved to be efficient in describing the low frequency dynamics of clathrate hydrate. Lattice dynamics calculations of xenon, methane hydrate with clathrate structure I (sI) reproduce successfully each feature position in the experimental spectra at acoustic band (blow 15meV) and yield correct relative intensity. Based on that, the assignment of the characteristic peaks for the vibrations of guest molecules/atoms in sI clathrates is determined. It is found that in the acoustic band in the spectrum of phonon density of states, two peaks with lower energy result from guest vibrations in large cages, while the features with higher energy result from their vibrations in small cages. Meanwhile, the broad shoulder in the methane hydrate case is explained by the overlapping of the original guest vibration features with those coupled from the host vibration, and the uncertain feature at ~6meV in the xenon hydrate case is assigned to anharmonic guest modes strongly coupling to small cages. Blue shift is proposed in phonon dispersion sheet in the case of anticrossing and found to be an evident symbol for guest-host coupling that explains the anomalous thermal conductivity of clathrate hydrate. This blue shift can be confirmed by observing different positions of the fingerprint in the previous IINS spectra of different gas hydrates under the same experimental condition, indicating varying strength of the coupling. The simulation results provide us information about the quantity, positions, intensity and the localizations of excitations of both host and guest molecules/atoms, which allows us to understand the features showed in the experimental results more insightfully.