Luminescence and Electrical Conductivity associated with Defects in Alkali Halide Crystals

Abstract

The emission and excitation spectra of the luminescence produced in potassium and sodium chlorides by thermal shock, mechanical strain or by the introduction of the divalent ions Ba, Sr or Ca show similarities in the same host crystal which are explicable on the assumption that the positive ion vacancies comprise or are part of the luminescent centre. Differences in the emission spectra in the same host crystal activated in these various ways are attributed differences of association of the vacancies. The electrical conductivity of pure annealed material as determined by an a.c. method is increased by thermal shock or mechanical strain. The similarity of the gradients of the graphs shows log conductivity plotted against reciprocal temperature of the thermally or mechanically strained samples with those of the annealed samples containing added divalent ions shows that the increased conductivity is due to positive vacancies; the magnitude of the increase as compared with the pure annealed crystals indicates that the number of `free' vacancies is increased some four five times by the thermal shock.