Extended Mie-Grüneisen theory applied to C60 in the disordered fcc phase.

Abstract

The large difference between the interatomic forces within a ${\mathrm{C}}_{60}$ molecule and the intermolecular forces allows the intermolecular contributions to thermodynamic properties to be treated by a simple pairwise central force theory when the molecules are freely rotating. This circumstance was utilized to compute the intermolecular contribution to the properties of the disordered phase. The anharmonicity was treated by an extension of Mie-Gr\"uneisen theory in which the Gr\"uneisen constant was replaced by a Gr\"uneisen parameter calculated as a function of volume from the intermolecular potential. The equation of state, compressibility, and coefficient of thermal expansion, as well as the ratio of the constant pressure to constant-volume heat capacity were computed in the quasiharmonic approximation using the calculated Gr\"uneisen parameter. The agreement with experiment was satisfactory on all counts.