Electrolytic coloration of the alkali halides with a special reference to the alkali fluorides

The kinetics of electrolytic coloration of the alkali halides is investigated. The first stage of the process takes place prior to the onset of coloration. It involves cation vacancy migration and creates conditions for the injection of electrons from an alkali metal cathode formed on the original pointed cathode. The injection of electrons into the alkali halides initiates a second stage (the electron-injection stage) which is characterised by the emergence of F coloration with a high concentration of F-centres near to the cathode. The concentration of F-centres is found to decrease linearly towards the anode. Electron injection takes place at the coloration starting temperature TQ which is found to be a characteristic of the alkali halide. Towards the start of the third or steady-current stage, a constant current density is established in the specimen and the concentration of F-centres is distributed homogeneously throughout the bulk of the specimen. The diffusion of F-centres in electrolytically coloured alkali halides is also studied. It is found that this process is governed by two mechanisms. The first, which dominates in the extrinsic range of temperature, is consistent with a diffusion process involving cation vacancy migration. At high temperatures (intrinsic range) the mechanism is thought to involve the self-diffusion of alkali ions. The successful application of the electrolytic technique to the production of homogeneous distribution of F and other colour centres in alkali fluorides containing mercury is also reported. It is shown, by means of ionic conductivity measurements corroborated with optical and other experimental data, that it is the divalent mercury cations which play the essential part in promoting electrolytic coloration. The effect of plastic deformation on electrolytic coloration is also investigated. It is suggested that dislocations present in the bulk of the specimens act as the principal source of excess anion vacancies. This suggestion is supported by optical as well as thermodynamic data. The influence of OH- ions on electrolytic coloration is found to reduce the colourability of mercury-doped alkali halides due to the formation of Hg2+ -OH- complexes. The optical properties of various electron and hole excess centres are reported for nine alkali halides. The effect of both thermal and optical treatment in the evolution of these centres is studied. It is deduced that electrolytic coloration is capable of a reproducible production of most of the known colour centres. The optical properties of uncoloured and electrolytically coloured alkali halides containing mercury are studied. The absorption spectre correlate with the ionic conductivity and thermal treatment results and indicate the formation of divalent, monovalent and neutral atomic centres. Two other centres, identified as (Hg - Hg)2+ and (Hg-Hg)0 , are also formed following thermal treatment and electrolytic coloration/ de-coloration respectively. Electrolytic coloration is found to change the valence of the mercury centres. An argument is presented for the formation of Hg- centres. [Please see abstract in the thesis for formulas