Thermodynamic Characterization of the Plastic Crystal and Non-Plastic Crystal Phases of C₇₀

Heat capacity measurements were performed on chromatographically purified C70 from 120 to 560 K by DSC. The experimental values were linked to the vibrational heat capacity, calculated based on normalmode vibration frequencies available from the literature. Good agreement is found, as prior for C60. Major orientational disorder is introduced in two closely spaced transitions, reflecting the change to the phase that displays anisotropic rotation of the rugby-ball shaped molecules. The total entropy of the disordering transitions is 22.4 J/(Kmol), slightly lower than that of C60, but additional entropy is gradually acquired between 120 K and the transition temperature, making the total entropy gain higher than that of C60, as expected for a less symmetric molecule. The Debye temperature for the six lattice vibrations of C70 was calculated to be 45 K, compared to 53 K for C60. No melting temperature could be detected for C70 up to 950 K, but the entropy of fusion is estimated to be 12 J/(K mol). Enthalpies, Gibbs energies, and entropies have been calculated from 0 to 1000K.