Lattice Dynamics Simulation Of Study Of Gas Hydrate

It is important to understand the formation and deformation processes and the stability limits of gas hydrates. The gas hydrate is stable only when guest molecules exist in the cages of the hydrate. In gas hydrates the guest-host interaction is weaker than the bond strength of cage lattice. However, the interactions between guest molecules and host lattice play an important role in the stability of the gas hydrates. Another interesting property of gas hydrates is the double occupancy of the large cages in gas hydrate. This paper presents the lattice dynamical (LD) simulation for hydrates containing Xe, Ar, and N2 guest atoms/molecules and calculated results were compared with the inelastic neutron scattering (INS) spectra. The host-guest interactions of various types of atoms/molecules or singly/double occupied were discussed. It can be found that two broad peaks (7.4meV and 10.4meV) are similar with the spectrum of empty-cage. These peaks are expected to be assigned to the fingerprint of the host lattice. In the results of Xe-hydrate, the frequency distribution is well reproduced by lattice dynamics calculations. Especially, the peak at 5.7meV is corresponding to the peak at 6.1meV of the experimental spectra, and its source has not been concluded because it was included in the range of the host lattice modes. Simulation results show that the interaction between the host lattice and Xe atoms in the small cages plays a more important role in the Xe-hydrate. In the case of N2 gas hydrate, two well-defined peaks at 2.0meV and 6.2meV are assigned to the vibrations of N2 molecules in large and small cages, respectively. Doubly occupied large cage locates at 5.6meV which is attributed to rotational motions of N2 molecules. This peak filled the vibration gap between small-cage and large-cage modes. The spectrum of Xe-hydrate is different from the results of N2-hydrate at low frequencies. We conclude from these observations that the guest molecular vibrations and guest-host interaction depend on the type of the guest atoms/molecules. Simulation results for N2-D2O hydrate reveal that the positions of guest vibrations are similar to N2-H2O hydrate. The increasing number of vibrational modes of N2-D2O hydrate is due to the difference in masses between hydrogen and deuterium.