Structure analysis of C-60 low-temperature phase by electron crystallography with cryo-TEM

The crystal structure of C-60 at liquid helium temperature was examined by the electron diffraction method using an imaging plate and cryo-TEM (transmission electron microscopy). The crystal of C-60 was SO thin that the electron scattering from this sample was able to be treated kinematically. However, the least-squares fitting among observed and kinematically calculated diffraction intensities resulted in an R factor of 0.23 for a structure model with only one major orientation. Similar large R factors are usually reported in the electron crystallography of thin crystals, in which a single perfect structure was assumed as a model structure. By considering structural disorders in the C-60 crystal, however, the R factor could be reduced to 0.12, when a minor crystal in a different orientation and also the f.c.c. (face-centered cubic) component were introduced to the model in addition to the major orientation crystal. Disorder in the crystal might be as important a factor as the dynamical scattering effect to be considered in electron crystallography for analyzing structures of thin crystals.