Self diffusion in cyclohexane single crystals—a re-appraisal

Abstract

Self diffusion in f.c.c. cyclohexane crystals has been re-examined using the radiotracer isotope mass effect technique. The experimental penetration curves show that this highly plastic solid is fundamentally very defective. Experiments for short periods yield overall diffusion coefficients dominated by diffusion along dislocations and mosaic boundaries (pipe diffusion) in the lattice. This line defect structure, which is known to increase with impurity content, is believed to be the cause of the very high values of the self diffusion coefficient found previously. The dominance of pipe diffusion was confirmed by isotope mass effect experiments carried out under the previous conditions. These gave values for the mass factor (ƒΔK)≃ 0.35 characteristic of pipe diffusion. Careful experiments for longer periods using ultra pure, well annealed, samples yielded values of the mass factor consistent with lattice self diffusion by a vacancy process. For these experiments the self diffusion coefficients and activation energy are in good relative agreement with values found for other rotator phase solids. The activation energy was confirmed by plastic deformation measurements.