Studies Of The New Phases For Methane Hydrates And Ultra Low-density Ices

Methane hydrate is solid crystalline inclusion compound formed by methane molecules and liquid water under certain temperature and pressure condition.It not only reserves abundantly in the deposits of permafrost,bottom of ocean and continental margins of the Earth,but also exists widely in the nebula and major moons of gas giants.Therefore,the research of methane hydrate is always the focus in the areas of energy,environment,climate,high pressure,and so on.Under different pressures,four clathrate methane hydrates(type ?,type ?,type H,and type K)and three filled-ice methane hydrates(type ?,type ?,and type ?)have been discovered by experiment or computer simulation.In chapter three,using Monte Carlo packing algorithm and dispersion-corrected density functional theory optimization with all the possible stoichiometric ratios of water molecules to methane molecules,we predicted type ?methane hydrate with stoichiometry of(H2O)3(CH4)which is dynamically stable.Even though it is a partial clathrate structure between the clathrate phase and filled-ice phase,its density and structural properties are close to the type ? filled-ice methane hydrate.To describe the relative stabilities of all the methane hydrates at different pressures,we constructed an H-P(formation enthalpy-pressure)phase diagram.The newly predicted type ? methane hydrate emerges as the most stable phase in the region between type ? methane hydrate and type ? methane hydrate.Besides,as pressure increases,the stable phase arises in the sequence of type K,type ?,type VI,and type ?.In conclusion,the discovery of type ? methane hydrate not only enriches the phase diagram of methane hydrates but also stimulates scientists to explore more possible new methane hydrate phases in the future.After the methane molecules are removed from the methane hydrates,a series of ultralow-density ices are obtained.As a result,our research focus turns to explore these new ultralow-density ices.Water is not only omnipresent on the Earth but also ubiquitous in the solar system such as on comets,asteroids,or icy moons of the giant planets.Hence,the exploration of different forms of ice in different environment conditions has important implication to chemical science,biological science,physical science,and planetary science.In chapter four and five,using dispersion-corrected density functional theory calculations as well as Monte Carlo and Molecular Dynamics simulations,we predicted two cubic clathrate ices with the ultralow mass densities and named them as s-? clathrate ice(p = 0.593 g/cm3)and s-IV clathrate ice(p = 0.506 g/cm3),respectively.The unit cell of s-? clathrate ice is composed of two large icosihexahedral cavities(8668412)and six small decahedral cavities(8248).Each unit cell of s-IV clathrate ice contains eight large icosihexahedral cavities(12464418),eight middle dodecahedral cavities(6646),and six small octahedral cavities(6246).The existence of large-sized icosihexahedral cavity and unique packing pattern of different cavities lead to the ultralow densities of s-? and s-? clathrate ices.By considering all the low-density(lower than ice ? or equal to ice ?)ice phases,a new P-T(pressure-temperature)phase diagram of water with TIP4P/2005 model was constructed under negative pressures.Below the deeply negative-pressure region of s-? clathrate ice,s-? and s-? clathrate ices replace s-H clathrate ice,arising as the most stable ice phases in the high-temperature part and the low-temperature part,respectively.As a result,a triple point(T=115 K,P =-4882 bar)appears in the phase diagram.The density functional theory calculations suggest that the s-? and s-? clathrate ices can be fully stabilized by encapsulating an appropriate guest molecule such as the dodecahedrane molecule(C20H20)and fullerene molecule(C60)in the large cavity,respectively.Considering that the guest-free s-? clathrate ice has been produced in the laboratory,which is also recognized as ice ??,both the s-? and s-? clathrate ices can be viewed as potential candidates of ice ?? or ice??.