On self-diffusion in plastic crystals

Abstract

Self-diffusion parameters for plastic molecular crystals are obtained by re-analysing nuclear magnetic resonance relaxation data using the theoretical treatment due to Wolf. This takes into account the correlated nature of the molecular motion. Using a model for vacancy induced motion previous anomalies in the N.M.R. interpretation are reduced and the parameters obtained using the various N.M.R. techniques are consistent for all plastic crystals studied. By taking account of the effects of correlated motion self-diffusion coefficients are calculated whose magnitude is generally in excellent agreement with those obtained by the radio-tracer technique, although some anomalies still exist. The improved agreement between different sources of experimental data demonstrates the existence of correlated molecular motion that arises, for example, with a vacancy-induced diffusion mechanism.