ISOTHERMS, THERMODYNAMIC PROPERTIES AND STABILITY OF THE FCC PHASE OFTHE C-60 FULLERITE - A THEORETICAL-STUDY

We are pursuing statistical-mechanical investigations of thermodynamic properties of the high-temperature modification of the C-60 fullerite taking into account the intramolecular degrees of freedom and the str ong anharmonicity of lattice vibrations. In our theoretical calculatio ns we employed the correlative method of an unsymmetrized self-consist ent field for strongly anharmonic crystals using the available experim ental information about the vibrational spectrum of this molecule. The Girifalco potential and the Yakub approximation are used. In the pres ent work we have calculated isotherms of this fullerite and a complete set of its equilibrium properties, including the components of the el astic tensors. Each isotherm has two branches V-1(P) < V-2(P), which c oalesce at some (minimal) pressure that depends on the temperature. At the left branch, B-T(P, V-1) > 0, while at the right one, B-T(P, V-2) < 0; the latter represents the absolute instability of the states V-2 (P). We have also investigated the thermodynamic stability of this ful lerite and the mechanism of loss of stability depending on the tempera ture. Our results for thermal and elastic properties are in good agree ment with experimental data available at low pressures and temperature s. (C) 1997 Elsevier Science Ltd.