Thermal expansion and lattice distortion of clathrate hydrates of cubic structures I and II

Thermal expansion of clathrate hydrates of argon, krypton, and propane with cubic structure II (CS-II), methane and xenon hydrates of cubic structure I (CS-I) and empty lattices of CS-I and CS-II at zero pressure have been investigated within the framework of lattice dynamics approach in quasiharmonic approximation. For all hydrates a good agreement with experiment for lattice parameters at some fixed temperatures have been obtained. In the case of the CS-II, it is found that inclusion of sufficiently small molecules such as argon and krypton into the water framework results in effective compression of empty hydrate lattice. In the case of large propane molecules included only in the large cavities the lattice is expanded relative to the empty lattice. The thermal expansion coefficients of hydrates with large enclathrated molecules are less than for hydrates formed by small guest molecules and the smallest value of thermal expansion coefficient is obtained for the empty lattice. By comparison of the data obtained for xenon and methane hydrates of CS-I and the empty lattice of CS-I it is found that the same behavior is observed also in the case of hydrates of CS-I. The effect of lattice stretching due to guest size on the reference chemical potential between the empty lattices of CS-I and ice Ih and empty lattice of CS-II and ice Ih is calculated too.