Temperature and pressure dependence of thermodynamic properties and stability of the C70 fullerite

Following the correlative method of unsymmetrized self-consistent field for strongly anharmonic crystals, we investigate thermodynamic properties of the high-temperature modifications of fullerites taking into account the intramolecular degrees of freedom. Here we calculate them for C70 fullerite as functions of the temperature and pressure up to 20 kbar. We use the potentials of Verheijen et al. and of Girifalco together with the available experimental information about the vibrational spectrum of this molecule. At any constant pressure its equation of state has two roots a1(T)<a2(T), which coalesce at some (maximal) pressure that depends on the temperature. At the lower branch, BT(T, a1)>0, while at the upper one, BT(T, a2)<0; the latter represents the absolute instability of the states a2(T). Along the lower branches of the isobars we calculate a complete set of equilibrium properties, including the components of the elastic tensors. We also study the thermodynamic stability of this fullerite and the mechanism of loss of stability depending on the pressure.