Annihilation of deformation-produced vacancy clusters in alkali halides at room temperature

The aging of plastically deformed crystals of NaCl and KCl at room temperature for a long time results in a bulk density increase as evidenced by thermal gradient flotation measurements. The effect is explained by the annihilation of deformation-induced clusters on dislocations. No density variation was found in LiF and NaF crystals. A model has been suggested for the formation of vacancy clusters by disrupture of unrelaxed dislocation dipoles. From the model it follows that cluster size and mobility should decrease in the homologic row of alkali halides, which is consistent with experimental evidence. The cluster diffusion coefficient has been estimated to be 10^?16 cm²s^?1 at room temperature.