Absolute stability boundaries of clathrate hydrates of cubic structure II

Stability of argon, krypton and propane clathrate hydrates of cubic structure II (CS-II) and of empty CS-II hydrate lattice has been investigated in a wide range of temperatures and pressures up to boundaries of absolute stability. The boundaries of mechanical stability in the framework of lattice dynamics method are determined in the quasiharmonic approximation from the Born stability conditions of crystal lattice. It has been shown by comparison of chemical potentials for empty hydrate lattices of structures I and II and ice Ih that at positive pressures empty hydrate lattices are thermodynamically metastable regarding to the ice Ih. At negative pressures there exist equilibrium lines between these phases. It has been found that despite weakness of the guest-host interactions, the guest molecules stabilize metastable host framework and the boundaries of stability depend on the type of the guest molecule. Moreover, in the cases of argon and krypton hydrates, the effect of double occupancy of the large cage on stability of CS-II structure has been also examined.