| Gas hydrate technology is one of research focus in recent years, and can be applied to solve problems in energy, environment, and other areas. Previous views had been that most gas hydrates decompose into gas and water (or ice) under 1 GPa or so, thus, research on hydrate structures in past few decades were conducted under low pressure conditions. Recent studies have shown the possibility that new structures of gas hydrate may exist under higher pressure, and this thesis aims to explore the structures of hydrate in carbon dioxide-water system and xenon-water system with the application of high pressure.;The evolutionary algorithm USPEX combined with first-principle calculation is applied in this study. The main research contents include the prediction of gas hydrate structures in certain conditions performed by USPEX codes, and the analysis of their physical properties. In the carbon dioxide-water system, structures are predicted under 5 GPa, by variable composition calculation. The results demonstrate that there's no thermodynamically stable structure of carbon dioxide hydrate under that condition. Properties of some typical structures (CO2˙7H2O, CO2˙4H 2O), which cannot stably exist in this system, are analyzed to explain the decomposition. In the xenon-water system, variable composition calculation are performed under 10 GPa, 20 GPa and 50 GPa, respectively. The results show the existence of metastable structure of xenon hydrate which can be indexed as 2Xe˙8H2O under 10 GPa. The data also illustrates there's no thermodynamically stable structures under 20 GPa and 50 GPa. To further study the structure of xenon hydrate, fixed composition calculation of 2Xe˙8H 2O is conducted under 5 GPa and 10GPa, and the physical properties of that structure are investigated and described in the thesis. |
