Abstract
Isotope-mass-effect measurements for self-diffusion in single crystals of pivalic acid in the plastic-crystal region (279·9–309·5 K) indicate that this process occurs by vacancy migration with some contribution from divacancies at high temperatures. Measurements at low temperatures (< 294 K) show some interference from pipe-diffusion processes.
The diffusion coefficients evaluated from these data and from supplementary measurements do not differ greatly from the previous measurements of Hawthorne and Sherwood (1970) and Brissaud-Lancin and Rivière (1979). The slight discrepancy with the former is attributable to the current use of material of improved purity. In the region which corresponds to vacancy diffusion, we evaluate an activation energy E d = 145 ± 15 kJ mol−1. This is relatively consistent with values for other molecular crystals.