B. T. Deshmukh

Additive coloration of microcrystalline powders of potassium halides

The additive coloration of potassium halide powders is reported. The optimum coloration temperature decreases with decreasing particle size. The stability of coloration was profoundly reduced in the powders in comparison with that in single crystals. The reduced stability of coloration is interpreted as due to the interaction of F centres with mobile dislocations whereas the decrease in the optimum coloration temperature is attributed to surface effects.

Correlation between microhardness, dislocation mobility and the isothermal stability of coloration in microcrystalline powders of KCl

Stable colour centres can be introduced in single crystals of alkali halides using electrolytic coloration. The powders obtained by crushing these single crystals lose colour very rapidly. Mobile dislocations which are present in far greater abundance in the powders are responsible for the reduced stability. Results on some KCl samples showing a correlation between microhardness, dislocation mobility and the stability of coloration are presented.

Isothermal decay of colour centres in microcrystalline powders of KBr:KI

Colour centres in single crystals are stable. The crystals can be stored in the dark for several months without any appreciable loss of colouration. The colouration of the microcrystalline powders, on the other hand, was found to decay very fast. This fast decay has been attributed to the defects inherent in the microcrystallites. The microcrystalline powders of the mixed crystals of KBr:KI could, surprisingly, retain their colouration for several days.

Isothermal decay of colour centres in microcrystalline powders of alkali bromides

Abstract The bleaching of F-centres on storing in darkness of electrolytically coloured single crystals and microcrystalline powders of solid solutions of KBr with other alkali bromides is reported. The colouration of the microcrystalline powders decays isothermally while that of single crystals is comparatively stable. The decay proceeds slower in the powders of mixed crystals than in the pure materials. A tentative explanation is suggested.

Colloid formation in microcrystalline powders of alkali halides

It is reported that the growth of colloids in NaCl powders is easy as compared with that in single crystals. This ease is attributed to the availability of more nucleation sites in the powders and to the motions of the colour centres being aided by dislocations. It is suggested that the study of colloids in microcrystalline powders provide information on such motions.

Etching studies of the non-dislocation pits in calcium-doped NaCl-KCl crystals

Mixed NaCl-KCl crystals doped with calcium were grown starting from the concentrated aqueous solution. Etchant consisting of ethanol+PbCl2 till saturation is found to attack selectively the dislocation sites. Etchant consisting of glacial acetic acid (99%)+FeCl3.6H2O solution was found to attach the non-dislocation sites preferentially. The dislocation and the non-dislocation pits were compared and the two types showed clear and definite distinction in etching behaviour. The cationic radii of the constituent salts and that of the dopant being nearly the same, dopant ions replace the host ions forming substitutional solid solution. From etching behaviour of the two types of pits and the comparison with that of the undoped sample, it is shown that the non-dislocation pits are at the sites of Ca2+ ions in the structure. By this one obtains a direct and ready means of locating Ca2+ ion sites in the host lattice without any sophisticated equipment.

On the growth of Z1 centres in microcrystalline powders of γ-irradiated KCl:Ca: the effect of particle size

The effect of particle size on the growth of Z1 centres in KCl:Ca is studied with the help of thermoluminescence measurements. The Z1 centres are more easily formed than F centres at higher concentration; this formation is achieved by reducing the particle size. It is argued that the dislocation densities and mobilities are the primary factors governing the growth of Z1 centres.

Thermoluminescence of Z1 centre in Sr-doped KCl

The formation of Z1 centres in KCl:Sr single crystals as well as microcrystalline powders is studied with the help of optical absorption and TL measurements. Upon F light bleaching of an irradiated crystal, a new glow peak at 405K is introduced in the glow curves. The peak at 405K is attributed to Z1 centres. A similar peak observed at 385K in microcrystalline powders is attributed to the combined action of deformation and Z1 centres. Their results led to the conclusion that Z1 centres are formed along with the F centres during irradiation and even the mechanical bleaching of F centres is sufficient to produce an appreciable number of Z1 centres without any optical bleaching.