DIFFUSION IN MOLECULAR-CRYSTALS

In recent years considerable attention has been focussed on molecular motion in solids. In particular the study of diffusion in molecular crystals has progressed with the development of n.m.r. and radiotracer techniques and a substantial literature has now built up. Recent theoretical developments in the analysis of n.m.r. relaxation and refinement of the radiotracer experimental techniques have allowed comparisons to be made between studies at the microscopic and macroscopic level. While some anomalies still exist, a general picture of the diffusion process in molecular crystals involving a vacancy mechanism has formed. The energy barriers to motion for many materials together with activation volumes for some are known with resonable certainty and various attempts to correlate these with thermodynamic parameters have been made. In this review, the various experimental techniques used for diffusion studies are described and a comprehensive survey conducted of the results of the investigations that have been reported in the literature. The most commonly studied materials are those that exhibit a platic-crystal phase. An attempt has been made to discuss the meaning and significance of what is currently known about diffusion in molecular crystals and the point defect structure of these solids. © 1979.