Theoretical study of thermodynamic properties of a family of fullerites from C-36 to C-96 in the equilibrium with their vapors
On the basis of the correlative method of the unsymmetrized self-consistent field that yields the account of the strong anharmonicity of the lattice vibrations, it has been calculated the temperature dependence of saturated vapor pressure of higher and smaller fullerites, from C-36 up to C-96, and their thermodynamic properties along their sublimation curves. We have used the intermolecular potential of Girifalco with parameters recently calculated for these fullerenes. The calculations were accomplished up to the temperature of loss of stability (spinodal point) T-s. We compare our results with available experimental data and with quantities calculated earlier for the magnitudes of the most widespread of the fullerites, the C-60. The behavior of some characteristics is considered in their dependence on the number of atoms in the molecule. The saturated vapor pressures up to the spinodal points of the two-phase systems crystal-gas is approximated by the formula log P-sat = A - (B/T) - CT, where the last term is related to the anharmonicity of the lattice vibrations. The coefficient A practically has no dependence on the number of atoms in the molecule (varying only by 2.2%); B increases monotonically, while C decreases from C-36 to C-96 by approximately twice. The isothermal bulk modulus B-T and the shear modulus C-44 vanish at the spinodal points.