Nitrogen Hydrate Cage Occupancy and Bulk Modulus Inferred from Density Functional Theory-Derived Cell Parameters

Gas clathrate hydrate solid materials, ubiquitous in nature as found either on the ocean floor, permafrost on the Earth, or in extraterrestrial planets and comets, are also technologically relevant, for example, in energy storage or carbon dioxide sequestration. Nitrogen hydrate, in particular, is of great interest as a promoter of the kinetics of the methane replacement reaction by carbon dioxide in natural gas hydrates. This hydrate may also appear in the chemistry of planets wherever nitrogen constitutes the majority of the atmosphere. A fine understanding of the stability of this hydrate under various thermodynamic conditions is thus of utmost importance to assess its role in the many fields where it is used. In the present work, we have investigated the structural properties of nitrogen hydrate by means of density functional theory calculations. We show that the lattice parameters strongly depend on the cage occupancy and that sI structure has higher bulk elasticity than the sII structure. An energy analysis reveals the key role played by the cage occupancy on the type of hydrate structure formed, which could be used for experimentally estimating the cage occupancy through the lattice parameter measurement.