Equation of state and thermodynamic functions for the fee soft-core multiple-Yukawa solid and application to fullerenes with compressible molecular radius

摘要

The generalized free volume theory is applied to the soft-core multiple-Yukawa solid, and the hard-core multiple-Yukawa is included in as a special case. The expressions for equation of state and internal energy are derived. The formalism developed is applied to the C_(60), C_(76), and C_(84) solids. The effective diameter of C_(60) molecule is taken as the experimental value; the parameters of the double Yukawa (DY) potential for carbon-carbon atoms are determined through fitting the experimental data of cohesive energy, the lattice constant, and the compression curve of C_(60) solid at ambient temperature. The effective diameter of C_(76) and C_(84) molecules are determined through fitting the experimental lattice constants at ambient temperature. The numerical results of C_(60) solid from the soft-core DY potential are in good agreement with the experiments, including the lattice constant and compression curve. The lattice constant versus temperature relationship for C_(76) and C_(84) solids calculated from the DY potential is qualitatively in accordance with experimental data as same as the Girifalco potential. The compression curve of the C_(84) solid calculated from the DY potential deviates from and is softer than the experimental data available. The reason for deviation is discussed, and it is concluded that the influence of compressibility of fullerene molecules to thermophysical quantities is important at high-pressure conditions.