The creation of point defects in alkali halides

Abstract

Three aspects of the creation of defects in alkali halides with ionizing radiation have been considered: (i) the mechanism by which Frenkel pairs are initially created, (ii) the effect of the thermal stability of the interstitial on the temperature dependence of the defect formation energy and the final equilibrium configuration of point defects, and (iii) the effect of radiation on the equilibrium between F -centers and F -aggregate centers. It has been suggested that a large body of experimental evidence indicates that Frenkel pairs are created in the negative ion sub-lattice by the ejection of a halogen interstitial in a replacement sequence along a [110] direction away from the vacancy. At low temperatures the nature of the Frenkel pair resulting from this process seems to depend on the magnitude of the Coulomb interaction between the vacancy and the interstitial and hence their final separation in the lattice. Since simple interstitial forms become unstable at relatively low temperatures the stability of interstitials at higher temperatures, as indicated by volume expansion and lattice parameter measurements, is associated with the formation of various halogen “molecular” species in the crystal matrix. These “molecules” may be associated with point defects and impurities in the positive ion sub-lattice and many of the effects due to the addition of divalent positive impurities seem to be related to such associations. It has been suggested that the temperature dependence of the equilibrium defect configuration arises from the need to form different types of stable “molecule” at various temperatures and not from some change in the fundamental mechanism of radiation damage. From this point of view the various “stages” of defect production arise from a saturation of a particular mechanism of interstitial stabilization and a consequent change in the relative importance of an alternate stabilization process. This would suggest that from the point of view of the vacancy only one “type” of F -center exists, and that many of the observations indicating more than one type of F -center is present could more properly be associated with the presence of a different interstitial species. Consideration of the negative-ion vacancy centers indicated that in X-irradiated crystals these existed in equilibrium concentrations determined by two processes: (a) the migration of previously created vacancies and (b) the radiation production and destruction of vacancies via the ejection and capture of interstitial halogens. The first process has an activation energy of about 0.6 eV and is essentially inoperative below 200°K. It seems that the migrating defect has the vacancy configuration of an α-center and that the aggregation with an F -center is controlled by an electron exchange interaction between the two defects. The second process is consistent with the favored model of vacancy creation via the ejection of an interstitial in a replacement sequence and its lower activation energy is close to the measured value for interstitialcy migration.