Specific heat study of C₆₀ single crystals

Abstract

We present the results of measurements of specific heat between 77K and 300K on a collection of C₆₀ single crystals, using quasi-adiabatic heat pulse calorimetry. In the temperature warming cycle the well known orientational transformation occurs at261.5 K, with a peak value of 16 J/gm.-K, both the temperature and magnitude being the highest reported. The transformation occurs at 260.4 K in the cooling cycle, the hysteresis being noticeable due to the extreme sharpness of the peak. Considerable excess specific heat, observed over the contribution due to intramolecular vibrations, is shown to arise from the molecular reorientations between two configurations, corresponding to a pentagonal or hexagonal face of a molecule facing the interhexagonal double bonds of the adjacent molecule. A hysteresis in C_p is also observed below 240K between the warming and cooling cycles, the termination of which at 100 K is identified with the orientational glass transition. Further, heat evolution is observed at many temperatures below 180 K. These features may indicate a complex orientational dynamics of the C₆₀ molecules.