Microstructural and optical changes induced by X-ray irradiation of single crystals of sodium chlorate
Crystals of sodium chlorate, exposed to various levels of X-ray irradiation from a synchrotron source, have been examined using X-ray topography, optical microscopy and UV–VIS spectroscopy. Two principal effects were observed during irradiation. First, a yellow coloration due to increased optical absorption with peaks at 290 and 420 nm was detected after an X-ray dose of 0.5 MRad. This has been attributed to the formation of radiation-induced colour centres. Secondly, after an accumulated dose of 3.1 MRad, significant changes in X-ray topographic images were observed. These comprised of an increase in integrated intensity and a vertical compression of the images. Both effects are due to sample curvature (convex towards the X-ray source) resulting from depth-dependant generation of point defects during irradiation. Throughout the investigation no development of new dislocations was observed and no changes were detected in the configurations of existing dislocations. Over a period of 2 to 3 weeks after irradiation, substantial (but incomplete) relaxation of the sample curvature was observed, together with partial bleaching of the optical absorption. In addition, numerous cavities formed within the crystals, particularly in the vicinity of solvent inclusions. All of these changes occurred at room temperature (25°C) and again no variation in the density or configuration of dislocations was observed. The defect-related processes potentially responsible for the observed behaviour are discussed. While some interdependence between optical bleaching and either curvature relaxation or cavity growth cannot be ruled out, it is apparent that two completely different defect types are associated with the latter two effects.