Abstract
Natural gas hydrates are ice-like crystalline materials formed from natural gas and clathrate ice under high pressure and low temperature. Ice XVI, the first S-II type clathrate ice produced in the lab, was simulated by first-principles density functional theory with the CASTEP code. A 34-molecule supercell was built to mimic the hydrogen-disordered structure. The vibrational spectra were calculated as a reference for inelastic neutron scattering (INS), infrared (IR) absorption, and Raman scattering experiments. Two kinds of H-bond vibration modes corresponding to two different bond strengths were found in our previous studies. In this paper, the statistics of distribution calculated by integrating these two kinds of modes was found to match the phonon density of states (PDOS) very well. We confirmed that the two basic types of H-bonds also appeared in clathrate ice XVI. The typical normal modes were analyzed to illustrate the dynamic process of lattice vibrations.
Highlights
Ice has a diverse presure temperature (P–T) diagram, with currently more than 17 crystal phases known under a variety of pressures and temperatures, including clathrate ice under negative pressure [1,2,3,4,5,6,7,8,9,10,11]
The resulting crystal, named ice XVI, has the lowest density of any member of the ice family found to date (0.81 g/cm2 ) and its discovery opened up research into negative-pressure clathrate hydrates
inelastic neutron scattering (INS) signals are proportional to phonons occur only near the center of the Brillouin zone (BZ)
Summary
Ice has a diverse presure temperature (P–T) diagram, with currently more than 17 crystal phases known under a variety of pressures and temperatures, including clathrate ice under negative pressure [1,2,3,4,5,6,7,8,9,10,11]. Natural gas hydrates are crystalline compounds composed of clathrate ice and natural gas. It was thought that empty hydrated lattices could not exist because guest molecules were needed to stabilize the host framework [17,18,19]. In 2014, Falenty et al experimentally prepared a clathrate ice crystal by evacuating the S-II type cage hydrate [10]. The resulting crystal, named ice XVI, has the lowest density of any member of the ice family found to date (0.81 g/cm2 ) and its discovery opened up research into negative-pressure clathrate hydrates. Ice XVI was later reported to have good mechanical stability and negative thermal expansion [22]. The typical vibration modes are discussed to clarify the dynamic process of lattice vibrations
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