ac Dielectric Investigations of the Gyulai-Hartly Effect: Enhanced Conductivity in Alkali Halides following Plastic Deformation

The real and imaginary parts of the dielectric constant of single-crystal specimens of NaCl have been measured as a function of time after plastic compressive strain of about 3%. Measuring frequencies ranged between 1 and 100 kHz and temperature between 20°C and 120°C. Both the real and imaginary part of the dielectric constant pass through a maximum at times after deformation which decrease with increasing temperature. A detailed kinetic analysis of the increase indicates that second-order kinetics are involved, and that the migrating defect has an activation energy of 0.74±0.01 eV, an energy characteristics of the cation vacancy. A similar kinetic analysis of the decrease indicates that first-order kinetics are involved with an activation energy of 0.64±0.05 eV. A model based on the assumption that isolated impurity ${\mathrm{Ca}}^{++}$ ions and cation vacancies are produced from clusters or precipitates by dislocation motion, and that the increase in the dielectric constant arises from the ${\mathrm{Ca}}^{++}$-cation vacancy dipole formation, while the subsequent migration of the ${\mathrm{Ca}}^{++}$-vacancy dipoles to the dislocations results in the observed decrease in dielectric constant satisfactorily accounts for the major experimental observations. The extrinsic dependence of the effect is verified by analytically extracting the Debye behavior of the ${\mathrm{Ca}}^{++}$-vacancy dipoles. The dielectric properties of old dislocations are shown to consist of a dipolar component of relative displacement of charge cloud and dislocation core and a purely ohmic component of ionic diffusion in the dislocation core.