TRANSMISSION ELECTRON-DIFFRACTION OF THE ORDERING TRANSFORMATION IN CRYSTALLINE C-60

Transmission electron diffraction is used to define the ambient-temperature structure of sublimated pure C60 crystals and examine the ordering transformation that occurs below 249 K. Convergent-beam electron-diffraction analysis of the symmetry of the [100] pattern for the ambient-temperature phase indicates an Fm3mBAR structure. Analysis of [100], [110], and [111] high-symmetry zone-axis diffraction patterns of the low-temperature ordered phase indicate that the structure belongs to space group Pa3BAR. Crystals with [111] (hexagonal plates) as well as those with other surface normals (rectangular [110] or [100] crystals) were found in the sublimated material. Electron-energy-loss near-edge fine structure demonstrates that significant bonding changes do not accompany the ordering. A twin with a (110) habit plane is identified in the ordered phase.