Self-Diffusion in Plastic Crystals

Self-diffusion coefficients of cyclohexane, neopentane, hydrogen sulfide, and hydrogen chloride have been measured in the plastic crystal phase just below the melting points by pulsed-gradient, spin-echo NMR. Except for a study of solid He3 by Reich and Garwin this is the first measurement of self-diffusion in a solid substance by spin-echo NMR. For all substances D ≈ 3 × 10-9 cm2/sec, which is about four orders of magnitude lower than the diffusion coefficients in the corresponding liquids just above the melting points. These results are in good agreement with values predicted from diffusional correlation times derived from spin relaxation measurements in the solid phases.