1.3 Structure and Properties of Fullerite

In an anologous situation to the solid noble gases, pure solid C₆₀ (fullerite) forms an FCC crystal (space group Fm$\bar{3}$m, Fig. 1.2) with relatively small van der Waals cohesive energy. Fullerite does not appear to melt, but rather sublimes at low pressure. In addition, the molecules in fullerite are arranged in an FCC lattice, and they freely rotate for temperatures above a structural phase transition (e.g. [26]) at T$_s \approx 257$K. The low temperature structure is no longer FCC but is simple cubic (space group Pa$\bar{3}$). In this structure, molecules with different orientations make up a larger unit cell than the FCC cell: the corner and face-centre molecules become inequivalent, i.e. there are four C₆₀ molecules in the crystal basis. However, the molecules in Pa$\bar{3}$ fullerite are not static. They continue to re-orient, but in a hindered way, jumping between minima of the orientational potential. These re-orientations gradually slow until the structure freezes (at about 90K) into an orientational glass whose equilibrium structure is the simple cubic Pa$\bar{3}$ orientationally ordered crystal.

Pure fullerite is a semiconductor with a bandgap of about 1.6 eV, a value reasonably close to the HOMO-LUMO gap of the free molecule.

Another interesting feature of fullerite is its instability to polymerization of the C₆₀ units. This polymerization can be caused by exposure to visible or UV light[27] or the application of hydrostatic pressure (e.g. [28]). The mechanism of polymerization is thought to be ``2+2 cycloaddition'', in which two double bonds on adjacent C₆₀ molecules break and reform as a pair of intermolecular single bonds, leaving a pair of single bonds in place of the broken double ones (see Fig. 1.3). Polymerized C₆₀ has very different properties than fullerite, e.g. it is very insoluble in toluene and has a very different vibrational spectrum[27].