Binary Phases of C60 and C70 Buckminsterfullerenes: An Electron Crystallographic Study

Abstract

Binary combinations of the fullerenes C60 and C70, in molar ratios spanning the concentration interval between the two pure components, were crystallized from dilute benzene solution to study their copacking in the solid state. From electron diffraction patterns, the hexagonal close packed structure was found to persist across the entire concentration interval, becoming more and more disordered when higher concentrations of C70 were present. The ordered face-centered cubic structure of C60, with crystals oriented along [111], disappeared when C70 was present at a molar concentration somewhat above 0.40. At high C70 concentrations (including the pure material), electron diffraction patterns indicated that a new primitive cubic structure, with a unit cell spacing a = 10.63 Å, might be present, even though such a molecular packing should not be energetically favored. It is shown, however, that such patterns can be produced when initially solvated three-dimensional crystals collapse to form a randomly displaced packing (shear direction: 〈110〉) of primitive square lattice layers as the solvent is rapidly evaporated. The disordered structure is, therefore, a distortion of the C70 fcc crystal structure oriented along [001].