ORIENTATIONAL GLASS TRANSITION AND STRUCTURAL RELAXATION IN SOLID C60

Abstract

We review recent experiments and models dealing with glass transition and the associated structural relaxation in solid C 60. This glass transition is thought to result from the freezing-in of thermally-induced dynamic orientational disorder in an otherwise orientationally-ordered crystalline structure. The structural relaxation is shown to be approximately exponential and linear (in the relaxation nomenclature), and the relaxation time nearly follows an Arrhenius temperature dependence over some 15 decades. C 60 is thus an example of an extremely ‘strong’ glass former in the ‘strong-fragile’ classification. The relaxation data are consistent with the simple structural model derived from X-ray and neutron diffraction, in which each C 60 molecule can be in two different, but energetically nearly equivalent (Δ≈10 meV ) orientational states, which are separated by an energy barrier of Ea≈250 meV . Small deviations from this simple picture are attributed to a slight temperature dependence of both Δ and Ea due to a changing local environment.