Stability and Composition of Helium Hydrates Based on Ices Ih and II at Low Temperatures

The recently developed approach describing host lattice relaxation, guest–guest interactions and the quantum nature of guest behavior (Belosudov, R. V.; Subbotin, O. S.; Mizuseki, H.; Kawazoe, Y.; Belosludov, V. R. J. Chem. Phys. 2009, 131, 244510) has been used to derive the thermodynamic properties of helium hydrates based on ices Ih and II. The p–T phase diagrams of the helium hydrates in different ices are presented for a wide range of pressures and temperatures, and the structural transitions between pure ice Ih and ice II as well as between ice Ih-based helium hydrate and ice II-based helium hydrate have been found to be in agreement with the available experimental data. The “ice II-based helium hydrate–ice Ih-based helium hydrate” equilibrium shifts toward the higher pressures in comparison with the line of “ice II–ice Ih” equilibrium. The degrees of interstitial space filling by helium in ice Ih-based and ice II-based hydrates decrease with increasing temperature and lowering of pressure. It is de...