The photochemical and thermal formation of colour centres in potassium azide crystals

Abstract

When freshly crystallized potassium azide is irradiated with ultra-violet light, colour centres are developed and the salt simultaneously decomposes with evolution of nitrogen and deposition of potassium in the lattice. The coloration has been investigated spectrophotometrically in the temperature range —196 to 300° C, and, by analogy with the colour centres present in irradiated and additively coloured potassium chloride, a detailed picture of the photochemical and thermal processes occurring has been obtained. Thus, at —196° C, F - and V -centres are formed, accompanied by the production of small F -centre aggregates at dislocations and sub-grain boundaries. The F -centres combine with some of the F -centres at — 78° C and at 0° C the remaining F -centres aggregate at dislocations to form R' -centres. These latter are converted at 60° C into filaments of metallic potassium, of thickness 2 or 3 atoms, distributed in the crystal and, at 276° C, clusters or colloidal centres, which are identified with the nuclei formed in the thermal decomposition of azides, are produced. Extension of the investigation to compressed pellets and evaporated films provides confirmation of these conclusions. It is further shown experimentally that the colorability of azide crystals is markedly affected by ageing and a mechanism for the ageing process is proposed.