Prediction of fullerene packing in C₆₀ and C₇₀ crystals

RECENT breakthroughs^1,2 in synthesizing large amounts of C₆₀, C₇₀ and other fullerenes³ have made possible studies of the struc-tures and properties of fullerene crystals. Using a force field developed recently for sp ² carbon atoms, we predict here the crystal structures and cohesive energies for close-packed crystals of C₆₀ and C₇₀. We predict, and confirm from calculations, that for C₆₀ face-centred cubic (f.c.c.) packing is more stable than hexagonal close-packing (h.c.p.), by 0.90 kcal mol^-1, whereas for C₇₀ h.c.p. is more stable than f.c.c. by 0.35 kcal mol^-1. The cubic structure of C₆₀ undergoes an orthorhombic distortion to space group Cmca at 0 K. At higher temperatures there is rapid reorientation (but not free rotation) of C₆₀ molecules, suggesting that above about 200 K a phase transition occurs to an orientationally disordered, f.c.c. structure (with a room-temperature lattice parameter of 14.13 Å). This may correspond to the first-order transition observed⁴ at 249 K. The threefold axes of the C₆₀ molecules in the low-temperature structure are not aligned with the threefold crystallographic axes.