ELASTIC-MODULI CALCULATION AND INSTABILITY IN STRUCTURE-I METHANE CLATHRATE HYDRATE

The thermal expansion of structure I clathrate hydrates with empty cag es and with enclathrated methane molecules has been calculated from 5 to 270 K using the lattice dynamics approach within the quasiharmonic approximation. The temperature dependence of several elastic propertie s. namely the dynamical, adiabatic and isothermal elastic moduli, are calculated. The dynamical and thermodynamical stability of the crystal s are investigated according to the Born stability criteria. Methane h ydrate is unstable against small homogeneous deformations near the mel ting transition at normal pressure. For the empty hydrate lattice, a t hermodynamical instability was found to occur at significantly higher temperature. The theoretical results are compared with new thermal exp ansion data on methane hydrate and results from a previous molecular d ynamics study. (C) 1998 Elsevier Science B.V.