Isotherms, thermodynamic properties and stability of the FCC phase of the C60 fullerite: A theoretical study

We are pursuing statistical-mechanical investigations of thermodynamic properties of the high-temperature modification of the C60 fullerite taking into account the intramolecular degrees of freedom and the strong anharmonicity of lattice vibrations. In our theoretical calculations we employed the correlative method of an unsymmetrized self-consistent field for strongly anharmonic crystals using the available experimental information about the vibrational spectrum of this molecule. The Girifalco potential and the Yakub approximation are used. In the present work we have calculated isotherms of this fullerite and a complete set of its equilibrium properties, including the components of the elastic tensors. Each isotherm has two branches V1(P) < V2(P), which coalesce at some (minimal) pressure that depends on the temperature. At the left branch, BT(P, V1) > 0, while at the right one, BT(P, V2) < 0; the latter represents the absolute instability of the states V2(P). We have also investigated the thermodynamic stability of this fullerite and the mechanism of loss of stability depending on the temperature. Our results for thermal and elastic properties are in good agreement with experimental data available at low pressures and temperatures. © 1997 Elsevier Science Ltd.