Lattice Dynamical Simulation Of Gas Hydrates

In the first section, a brief overview is provided about the general situation and the importance of gas hydrate. And then fundamental theories and methods of computer simulation techniques are expatiated. On the basis of the basic knowledge, we have launched a computer simulation study of gas hydrates.Lattice dynamical simulation of noble gas hydrate (He, Ne, Ar, Kr, Xe) and methane hydrate of structure I and II has been carried out focusing on their structure and dynamical characteristics. Comparisons in the lattice energy and vibrational mode of the included molecules are performed and to study the mutual influence between the host lattice and the guest molecules. It is found that the repulsive interaction between the guest molecules and the host structure plays an important role in the hydrate system.The PDOS spectrum of empty hydrate of structure I and II have been calculated under the same external condition and it was found that the two features resemble so much in both the translational region and the librational region. By introducing the guest molecules, differences in the dynamical characteristics between the two kinds of gas hydrates appeared. Potential energies of the lattice were investigated to study the influence of the guest species on the stability of the hydrate structure. Results show that the species of the included molecules determined the stability of the hydrate. when the diameter of the inclusion molecules is between 4.2? and 3?, such as Ar and Kr, the critical role of the 512 cage in the stabilization of hydrates becomes effective. Further we proposed that the fraction and the crystallographic position of 512 cages in the whole crystalline system should also be responsible for the thermodynamic stability of hydrates besides its microstructure emphasized by the conventional viewpoints. Meanwhile, some preliminary results on the hydrate included with non-spherical molecules have also been worked out. When methane molecules are filled in the structure II hydrate lattice, the lattice energy falls below the empty lattice energy. This indicates the possibility of the existence of structure II methane hydrate, although this kind of hydrate has not been discovered.On the other hand, we studied the influence of host lattice on the guest molecules. For hydrate of structure I and structure II, the vibrational modes of Xe molecules included in the large and small cages were calculated, respectively. This paper mainly discusses the the low-frequency part of the PDOS spectrum of Xe molecules included in the cage structure. With the help of lattice dynamical calculations, the modes attributions are identified directly. The calculated vibrational spectrum of Xe molecules included in the structure I hydrate is consistent with the inelastic neutron scattering spectrum observed by Baumert et al. The vibrational modes of Xe molecules included in the structure II hydrate are also calculated and two excitations which are different from that of structure I are observed. The results show that although the same Xe molecules are included in the two kinds of hydrate structures, the vibrational modes of Xe molecules in different lattice structures are not the same. The differences between the cage structures are responsible for the differences found in the PDOS spectra. And we proposed the resonant effect of fingerprint frequency of host lattice at about 7meV and 10meV which arise from the coupling of Xe molecules in the 512 small cage with the host lattice. Because of the higher fraction of the 512 small cage in structure II hydrate, the resonance effect between the guest and the host is found to be promoted comparing with that of structure I.