The heat capacity of solid C₆₀

High resolution heat capacity measurements of the prototypic fullerene, C₆₀, are presented between 13 and 300 K. The well-documented first-order phase transition is clearly observed at 257.6 K and is associated with enthalpy and entropy changes of 7.54 and 30.0 J K^-1 mol^-1, respectively. A more subtle transition is observed at 86.8 K. This transition is attributable to the onset of orientational glass behaviour related to the kinetics of molecular reorientation. The rate of the enthalpy relaxation at the glass transition is accurately reproduced by a simple exponential function. The Arrhenius parameters describing the temperature dependence of the relaxation time are E_a = 22.2 ± 1.0 kJ mol^-1 and τ₀ = 4×10^-11±1s. At low temperatures, solid C₆₀ is a glassy crystal in which molecular orientational disorder is frozen in as a consequence of the long relaxation time of the molecular reorientation.