STRUCTURAL PHASE-TRANSITIONS AND ORIENTATIONAL ORDERING IN C-7O

The thermal behavior of solid C-70 has been studied by synchrotron X-ray powder diffraction and differential scanning calorimetry. The equilibrium solid state structures formed by C-70 were solved by full profile refinement techniques in which orientational and packing disorder were explicitly accounted for. Above 345 K, C-70 forms a plastic crystal, with an equilibrium face-centered cubic structure. At lower temperatures, orientational freezing occurs in two stages. Between 295 and 345 K, disorder persists only about the long axis of the molecule, and the lattice undergoes a rhombohedral distortion. Below 295 K, the rhombohedral lattice undergoes a further distortion, resulting in a previously unobserved monoclinic structure, in which the molecules are presumed to be essentially static. At all temperatures, however, the structure of C-70 retains an ABC packing sequence.