THERMODYNAMIC CHARACTERIZATION OF THE PLASTIC CRYSTAL AND NONPLASTIC CRYSTAL PHASES OF C-70

Heat capacity measurements were performed on chromatographically purif ied C-70 from 120 to 560 K by DSC. The experimental values were linked to the vibrational heat capacity, calculated based on normal-mode vib ration frequencies available from the literature. Good agreement is fo und, as prior for C-60. Major orientational disorder is introduced in two closely spaced transitions, reflecting the change to the phase tha t displays anisotropic rotation of the rugby-ball shaped molecules. Th e total entropy of the disordering transitions is 22.4J/(Kmol), slight ly lower than that of C-60, but additional entropy is gradually acquir ed between 120 K and the transition temperature, making the total entr opy gain higher than that of C-60, as expected for a less symmetric mo lecule. The Debye temperature for the six lattice vibrations of C-70 w as calculated to be 45 K, compared to 53 K for C-60. No melting temper ature could be detected for C-70 up to 950 K, but the entropy of fusio n is estimated to be 12J/K mol). Enthalpies, Gibbs energies, and entro pies have been calculated from 0 to 1000 K.