Short-range-order correlations in the orientationally disordered phase of hexachloroethane. II. Elastic and quasielastic neutron scattering

The dynamical origin of the orientational disorder in the plastic crystalline phase of hexachloroethane (C2Cl6) has been determined by inelastic neutron scattering. A comparison between the energy-integrated neutron scattering and the diffuse X-ray scattering, which was reported previously, enables single-molecule scattering and contributions to the diffuse scattering due to short-range correlations to be distinguished. A single-particle rotational diffusion model explains the quasielastic part of the inelastic scattering. The one-dimensional ordered clusters have a considerably larger characteristic time and contribute to the 'elastic' intensity. Reasons for the complete absence of well defined collective translational excitations are discussed.