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 ( f A K ) 2: 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.

NMR Investigation of Molecular Motion in the Neat Solid and Plastic-Crystalline Phases of Cyclohexane1

“…The scaling factors, 0.65 and 0.19, associated with the two averaged quadrupole interactions are suggestive of C−D bond angles corresponding to 29 ± 2° (perhaps for axial C−D bonds) and 133 ± 4° (perhaps for equatorial C−D bonds) with respect to a C 3 rotational axis. In this hypothesis, the corresponding D eq −C−D ax angle, 105 ± 5°, is within the limits of the published value, 107.5 ± 3° . Thus, the deuterium quadrupole echo NMR data indicate an onset at approximately 170 K of “rapid” rotation (e.g., 10 2 kHz) for a relatively small fraction of the cyclohexane- d 12 molecules in the solid phase.…”

“…However, this result cannot be rationalized from the physical nature of the problem, particularly with respect to the large offsets of the Gaussian peaks. The lack of a physically reasonable good fit is not surprising given the complex nature of cyclohexane dynamics in the plastic phase: cyclohexane is unique among low entropy-of-fusion plastic solids with regard to its extremely high concentration of “pipe defects”; self-diffusion within these defects is known to dominate over the bulk lattice process at low temperatures in the plastic phase and is believed to proceed by a complex jump mechanism involving, on average, 12−18 molecules. , …”

Plastic Crystal Electrolyte Materials: New Perspectives on Solid State Ionics

“…Pipe diffusion has been shown to be dominant in plastic crystals that have not been carefully annealed such that a high density of dislocation is present. [58,64] We have seen evidence of such behavior in electron microscope studies of P 13 BF 4 [66] where distinct rod shaped crystals appear in a background of apparently amorphous material. If one looks at the local structure of the crystals, defects can be clearly seen; Figure 9 shows a layered structure of approximately 50 nm layers, probably representing slip planes.…”

Dynamics of plastic and liquid cyclohexane in bulk and in porous glasses studied by NMR methods

“…This is the origin of the plastic deformations observed in this material [1][2][3]. It is therefore of interest to study translational diffusion by means of NMR techniques.…”