Lattice defects in plastic organic solids

A study has been made of nuclear magnetic resonance line-narrowing at temperatures approaching the melting point for several organic crystals in their plastic, rotator, phase. It was found that, whereas those solids with higher fusional entropies (>4·6 e.u.) yielded activation energies for the line narrowing process which were equivalent to those for self-diffusion as measured by radiotracer and plastic deformation techniques those with entropies of fusion <3·3 e.u. yielded energies which were much lower than those for self-diffusion. Solids with entropies in the intermediate region showed the former behaviour at low temperatures and the latter at high temperatures. It is proposed that this variation reflects a difference in the physical nature of the point defect in the various solids.