A study of interstitial clusters in irradiated alkali halides using direct electron microscopy

Abstract

A direct electron microscope study has been made of defect clusters produced in alkali halides by ionizing radiation. A technique has been developed for preparing foils of alkali halide crystals less than a few hundred nanometres thick for observation in the microscope, and a liquid helium cooled stage allows the study of irradiated crystals down to 5 K. At this temperature it has been possible to analyse in detail clusters produced at other temperatures, without confusion from observable damage produced by the investigating election beam. Irradiations have been performed both in the 100 keV microscope beam and externally usingγrays or 400 keV electrons. In this way irradiation products in KI have been extensively studied. The predominant type of defect cluster produced in several alkali halides by room temperature irradiation has contrast typical of planar dislocation loops. Tilt/contrast experiments performed in KI show conclusively that the loops are interstitial in character, and the numbers of point defects involved and the temperature dependence of their growth indicate that mobile interstitial halogen atoms are required for their formation. The loops have Burgers vectorsb═ 1/2a(110) where a is the lattice parameter. The habit plane of a loop withb═ 1/2a[101] lies between (001) and (101). An unfaulted interstitial dislocation loop with Burgers vector 1/2a<110> formed by condensation of equal numbers of anion and cation interstitials can account for the observed features of the clusters. However, since there is little evidence for the direct production of cation Frenkel pairs by ionizing radiation in alkali halides, alternative models are presented in which the cluster forms primarily as a result of the initial aggregation of halogen interstitials alone. These models are shown to be consistent with specific features of the radiation damage behaviour of alkali halides.