Statistical-mechanical investigation of the saturated vapor pressure, the enthalpy of sublimation and thermodynamic stability of the FCC phase of the C70 fullerite

Using the correlative method of the unsymmetrized self-consistent field for strongly anharmonic crystals and the virial expansion for a gas we have calculated the temperature dependence of the saturated vapor pressure P-sat and the enthalpy of sublimation Delta H-sub for the FCC modification of the fullerite C-70. The potential of Verheijen et al. [Science, 1992, 166, 287] with parameters calculated on the basis of theoretical estimates by Pan er al. [J. Phys. Chern., 1991, 95, 2944] and experimental data of Baker et al. [Chem. Phys. Lett., 1991, 184, 182] and Vaughan et al. [Science, 1991, 254, 1350] have been utilized for the intermolecular forces. The temperature of loss of stability has been estimated for the two-phase crystal-vapor system. A comparison has been made between the results obtained with different sets of parameters and the values calculated earlier for the C-60. The correspondence with available experimental data is satisfactory. Over the whole range of temperatures studied the dependence of the logarithm of the vapor pressure on the inverse of the temperature is close to linear: log P-sat approximate to A - B/T - CT, while Delta H-sub decreases about 20%. Using the scaling law an estimate of the triple point parameters is given for the C-70: T-tr approximate to 1500 K, P-tr approximate to 655 torr. (C) 1997 Elsevier Science Ltd. All rights reserved.