${\mathrm{C}}_{60}$ one- and two-dimensional polymers, dimers, and hard fullerite: Thermal expansion, anharmonicity, and kinetics of depolymerization

We report on high-resolution thermal expansion measurements of high-temperature-pressure treated ${\mathrm{C}}_{60}$ [one-dimensional (1D) and (2D) polymers and “hard fullerite”], as well as of ${\mathrm{C}}_{60}$ dimers and single crystal monomer ${\mathrm{C}}_{60}$ between 10 and 500 K. Polymerization drastically reduces the thermal expansivity from the values of monomeric ${\mathrm{C}}_{60}$ due to the stronger and less anharmonic covalent bonds between molecules. The expansivity of the “hard” material approaches that of diamond. The large and irreversible volume change upon depolymerization between 400 and 500 K makes it possible to study the kinetics of depolymerization, which is described excellently by a simple activated process, with activation energies of $1.9\pm 0.1\mathrm{eV}$ (1D and 2D polymers) and $1.75\pm 0.05\mathrm{eV}$ (dimer). Although the activation energies are very similar for the different polymers, the depolymerization rates differ by up to four orders of magnitude at a given temperature, being fastest for the dimers. Preliminary kinetic data of ${\mathrm{C}}_{70}$ polymers are presented as well.