THE THERMAL-PROPERTIES OF 4 ALLOTROPES OF CARBON

The heat capacities of diamond, graphite, and the fullerenes C-60 and C-70, are compared and linked to their approximate vibrational spectra . Because all the allotropes have the chemical composition C, their ul timate heat capacity is the Dunlog-Petit value 3R = 24.9 J K-1 mol(-1) . At low temperatures (0-50 K), the fullerenes have a much higher heat capacity than the other two allotropes. As the temperature is increas ed, graphite and the fullerenes approach approximately equal heat capa cities. Up to 1000 K, diamond has a lower heat capacity, but then, bec ause of its weaker C-C bonds compared to the conjugated double bonds i n graphite and the fullerenes, it exceeds the heat capacities of the l atter. Fullerenes have crystal to plastic-crystal transitions in the r egion 250-350 K with entropies in agreement with the orientational ent ropy increase derivable from Walden's rule. Their sublimation pressure s reach atmospheric pressure at about 1500 K, and the entropies of tra nsition, when corrected for the entropy of fusion (Richards' rule), ob ey Trouton's rule. Carbon is thus a textbook example for the differenc es in thermal behavior possible as a function of chemical bonding and structure.