Stability of color centers in microcrystalline powders of alkali halides

In the present paper, a theoretical approach has been made to the stability of color centers in microcrystalline powders of alkali halides. The diffusion of point defects from higher concentration region to lower concentration region may give rise to the unpinning of some of the pinned dislocations, whose interaction with the color centers may cause the capture of their electrons. The captured electrons may move with the unpinned dislocations and they may subsequently be transported to different trapping centers. This process may give rise to the decay of coloration in the microcrystalline powders. Considering the decay of coloration by the unpinning of dislocations, an expression has been derived for the coloration decay in microcrystalline powders of alkali halides. The decay of coloration is different for different alkali halide crystals and it is faster in the particles obtained by crushing the single crystals as compared to the particles of the same size obtained by cleaving the single crystals. The decay of coloration increases with decreasing size of microcrystals, increases with temperature of the crystals, decreases with increasing radiation dosage given to the crystals, and decreases with inclusion of dopants in the crystals. The coloration decay is faster in gamma-irradiated crystals as compared to the electrolytically colored crystals. The dependence of coloration decay of microcrystalline powders on different parameters may be understood using the theory based on the interaction of unpinned dislocations with the color centers.