Thermodynamic properties of the C-60 fullerite at high temperatures: Calculations taking into account the intramolecular degrees of freedom and strong anharmonicity of the lattice vibrations

Basing our calculations on the correlative method of the unsymmetrized self-consistent field for strongly anharmonic crystals, we study thermodynamic properties of the high-temperature (fcc) modification of the C-60 fullerite. In this work a complete set of its thermal and elastic properties have been calculated at normal pressure from the experimental equilibrium point with the low-temperature phase to the instability temperature. In its high-temperature phase, the molecules rotate rather freely and the noncentral part of the intermolecular forces almost disappears. Because of this, it can be treated as a van der Waals crystal with a lot of intramolecular degrees of freedom. To take them into account we have used available information about the normal modes of intramolecular frequencies and their degeneracies. For the intermolecular forces the Girifalco potential and its Yakub approximation have been utilized. The intramolecular vibrations have no effect on the equation of state and related properties but are crucial in specific heats. Our results are in good agreement with experimental data. The possible melting temperature of the C-60 fullerite and the behavior of its thermodynamic properties near the spinodal point are discussed. At this point the isothermal bulk modulus B-T and (unlike in familiar van der Waals crystals) the elastic constant C-44 go to zero, and the thermal expansion coefficient a and the isobaric specific heat C-p tend to infinity. Other stability coefficients remain finite and positive.