Theoretical study of thermodynamic properties of high-temperature phase of C-76 and C-84 fullerites

Theoretical investigations of thermodynamic properties of orientationally disorded phases of fullerites on the basis of the correlative method of unsymmetrized self-consistent field have been done. The results for two higher fullerenes - C-76 and C-84 at normal pressure are reported. The Girifalco potential with appropriate parameters is used. We have solved the equation of state that governs the nearest-neighbour distance in a crystal. We have calculated the isothermal bulk modulus, the isothermal elastic constants, the thermal expansion coefficient, and the Grueneisen parameter. The results are compared with those obtained previously for C-60 We discuss the temperature dependence of the stability coefficients and the mechanism of the loss of the stability at spinodal points T-S.