Low-Temperature Annealing of Irradiation-Induced Defects in LiF

The expansion of alkali halides due to x-ray irradiation has been observed at 90° and 300°K. Simultaneous measurements of the $F$ band indicate that the expansion can be roughly explained on the basis of the generation of negative ion vacancies. However, partial annealing of the expansion of LiF at approximately 130°K after irradiation at 90°K is not accompanied by a decrease in the density of negative ion vacancies associated with $F$ centers, therefore indicating that the expansion cannot be completely due to the increase in the density of $F$ centers. The low-temperature annealing of the expansion can be associated with either vacancy pair diffusion or interstitial motion. While experiments with plastic deformation and $M$ band formation in LiF argue against vacancy pair diffusion, the spin resonance experiments of Känzig and Woodruff support the interstitial interpretation. It is therefore tentatively concluded that halogen vacancy interstitial pairs are generated by x-ray irradiation in LiF at 90°K and that the low-temperature annealing is due to a change in interstitial configuration.