Structures, stabilities and phase diagram assessments of clathrate ices at negative pressures

Abstract

Clathrate hydrates are ubiquitous in terrestrial oceans or asteroids in the solar system, attracting the attention of scientists and forming an unstoppable research wave. Here, we study 9 ice phases of the empty clathrate hydrates sII, sIII, sIV, sH, dtc, EMT, sL, ITT and ice XI at negative pressures based on the second-order Møller-Plesset perturbation (MP2) theory and the higher-level CCSD(T) theory. A renewed phase diagram at negative pressures is proposed, where the empty hydrates sII, dtc, and EMT are demonstrated to be thermodynamical stable from −0.26 to −0.80 GPa. The predicted lattice parameters and Raman spectra of empty clathrate ices are in excellent agreement with the observable results, which validates the accuracy of the proposed methods in predicting the structures and phase diagram. The proposed work is crucial to the exploration of ice polymorphs and the scientific understanding of the properties of molecular crystals at negative pressures.