The structure of different phases of pure C70 crystals

Single crystal of pure C70 are grown from the vapour phase and the structure and morphology of these crystals is studied. By means of X-ray diffraction and TEM measurements five different phases are observed. The observed phases are (from high to low temperatures) fcc, rhombohedral, ideal hcp (c/a=1.63), deformed hcp (c/a=1.82) and a monoclinic phase. The occurrence of these different phases and the phase transitions is accounted for in a simple model. For the monoclinic structure a model for the stacking of the orientationally ordered molecules in the lattice is proposed. For both the hcp and fcc phases a Lennard-Jones type interaction potential is used to calculate bond strengths, lattice energies and the theoretical morphology.