Specific Heat of Powdered Buckminsterfullerene from 20 K to 300 K by Continuous-Cooling Calorimetry

A heat capacity study of polycrystalline C _{60} by continuous-cooling calorimetry is presented. Details of the continuous-cooling method and the experimental setup are described. The method was tested by measuring the heat capacity of solid copper sample, a powdered graphite sample and a solid glassy carbon sample and comparing to known results. Heat capacity measurements of loose powdered C_{60} in a He-filled envelope and of a pressed pellet of C _{60} powder mixed with copper powder were made between 20 K and 300 K. Evidence of a first order phase transition in solid C_{60} is shown clearly by the large heat capacity anomaly at T _{rm c} = 259 K, but no clear evidence emerged in this study of the glass transition at 90 K seen by other workers. By selecting baselines under the peak from 120 K to 300 K, the enthalpy and the entropy of the transition are estimated as 8.5 +/- 0.4 kJ/mol and 36.2 +/- 1.9 J/K-mol for the loose powder sample, and 7.6 +/- 0.6 kJ/mol and 33.7 +/- 3.1 J/K-mol for the pellet sample. The contributions to the heat capacity of C_{60} arising from vibrations, librations and rotations are calculated using standard models and experimental information. For the pellet sample in the regime above T_ {rm c} there is a deficit of about 20% between these contributions and the measured heat capacity. The presence of a substantial number of defects was confirmed by broadened and washed out Bragg peaks in an x-ray diffraction pattern of this pellet sample. With the aid of a simple phenomenological model, the contribution of the annealing of these defects to the heat capacity is calculated and used to explain the heat capacity deficit.