Dynamics and equation of state of hydrogen clathrate hydrate as a function of cage occupation

The recent discovery of hydrogen clathrate hydrate with potential for efficient storage of molecular hydrogen at modest pressure has initiated significant scientific interest. Of critical importance for the amount of hydrogen that can be stored is the number of hydrogen molecules that occupy the cages in the hydrate clathrate host structure. The experimental data of cage occupancy is controversial, and the reproducibility of the initial experiment has been questioned. Therefore, in the present report we study the dynamic and thermodynamic properties of hydrogen clathrate hydrate as a function of cage occupancy using lattice dynamics calculations in order to determine the most favored occupancy at different external conditions. Lattice dynamics investigations show that all hydrogen hydrates with multiple species per cage are dynamically stable and phonon spectra strongly depend on the cage occupancy. As a result all thermodynamic functions vary significantly with cage occupancy. It is shown that quantum zero-point vibrations are of fundamental importance for the thermodynamic properties. The equation of state has been estimated as a function of temperature and cage occupancy.