Crystal structure and bonding of ordered C₆₀

STRUCTURAL studies^1-.3 of crystalline C₆₀ (ref. 4) have indicated that at room temperature the C₆₀ molecules are orientationally disordered and the crystal structure may be regarded as a face-centred cubic configuration of C₆₀ spheres. Below 249 K, however, the molecules become orientationally ordered³ and a simple cubic lattice results, corresponding to a symmetry change from Fm3̄ to Pa3̄. Here we present the results of a neutron powder diffraction study of the low-temperature ordered structure, which reveals the packing configuration of the C₆₀ molecules. The C₆₀ units are rotated in an anticlockwise manner around the [111] direction by ∼98°from the ideal Fm3̄ configuration. This apparently arbitrary rotation in fact results from an optimized ordering scheme in which electron-rich short (1.391-Å) inter-pentagon bonds face the electron-poor pentagon centres of adjacent C₆₀ units. The high symmetry of the C₆₀ molecule allows these interactions to be optimized identically for all twelve nearest neighbours, a possibility that is by no means intuitively obvious. The bonds common to a given pentagon are somewhat longer (1.455 Å). The high degree of bonding optimization and the absence of bonding frustration accounts for the high ordering temperature of 249 K (ref. 5).