Thermodynamic properties of some polymeric forms of fullerite C60 at temperatures ranging from 0 to 320 K

Abstract

The temperature dependences of the heat capacityC 0 p of fullerites C60 were studied at temperatures ranging from 5 to 320 K in an adiabatic vacuum calorimeter with an accuracy of 0.4–0.2%. The fullerite C60 samples were prepared by treating the starting fullerite C60 under 8 GPa at 920 and 1270 K and “quenched” by a sharp decrease in pressure to −105 Pa and in temperature to ∼300 K. Fullerite C60(8 GPa, 920 K), a crystalline polymer with layered structure formed by polymerized fullerene C60 molecules, was obtained at 920 K and 8 GPa. Fullerite C60(8 GPa, 1270 K), a three-dimensional polymer with a graphite-like structure formed by fragments of decomposed C60 molecules and containing many C(sp3)−C(sp3) bonds, was obtained at 1270 K and 8 GPa. Both polymers are metastable polymeric phases. The anomalous character of the temperature dependence of the heat capacity was revealed in the 49–66 K range for the polymer formed at 1270 K. The thermodynamic functions of the substances under study were calculated for the 0–320 K region along with entropies of their formation from graphite. The entropies of transformation of the starting fullerite C60 into metastable phases and that of intertransformation of phases were estimated.