Mechanism for Dislocation Pinning in the Alkali Halides

Abstract

Changes in the elastic modulus and internal friction of plastically deformed rocksalt, caused by changes in the mean free length of oscillating dislocation segments, have been observed during x-irradiation at temperatures ranging from 10° to 300°K. The rates at which the modulus and damping changes occur during irradiation are found to be independent of temperature; but during irradiation at helium temperature, the magnitude of the effects are smaller. This decrease in magnitude has been shown to be due to the absence of thermal unpinning. Explanations of the pinning process based on the diffusion of point defects to dislocations appear to be ruled out by the rapidity of the low temperature modulus change. The dislocation pinning produced by irradiation of NaCl, KCl, KBr, and KI at low temperatures is found to be reversible when the crystals are subjected to illumination in, or near, the visible. The wavelength of the illumination which produces the maximum unpinning increases as the lattice constant increases; the effect disappears when the crystals are warmed to room temperature. However, at all temperatures, bleaching a deformed and partially irradiated crystal with F light causes a further modulus increase. The pinning point produced by low temperature irradiation is identified as a dislocation jog in close association with an F center whose absorption band is displaced towards the red by the dislocation strain field. Ionization of the F center results in recombination of its vacancy with the jog; warmup allows the F center to diffuse beyond recombination range.