Modeling of phase transformation sII-sH in Ar hydrates at high pressure

Dynamical and thermodynamical properties of Ar hydrates sII and sH with multiple filling of the large cages are studied within the framework of the lattice dynamics approach in the quasiharmonic approximation. The approach is based on previously suggested generalization of the van der Waals and Platteeuw theory of clathrate hydrates. The existence of structural transition from sII to sH structure in Ar hydrates at elevated pressure is shown. The calculation of Ar structure sH hydrate with single occupancy of medium and small cages and occupancies from 1 to 5 of large cages shows thermodynamic stability of these phases and agrees with the experimental data.