Some optical properties of the alkali halides containing divalent impurities

Abstract

The investigation was limited mainly to NaCl.Mn++ and NaCl.Ca++, and was concerned with colour centres in single crystals of these materials. The crystals were grown by the Kyropoulis technique and optical absorption measurements were made with a grating spectrophotometer. The emphasis of the work was placed on the production of Z-bands in NaCl. Ca++ but the effects of optical and thermal bleaching and of quenching were investigated in all crystals. The results of the work on NaCl.Mn++ confirmed the model of Schneider and Caffyn (1955) in which the Mn++ ions and positive ion vacancies are deposited at dislocations and become more uniformly distributed after quenching. In particular, it was found that quenched crystals were luminescent after being X-rayed, indicating the presence of dispersed Mn++ ions; a similar result was found in NaCl .Ni++. A short study of the distribution of Mn++ in crystals was also made. It was found that Mn++ and Ni++ gave rise tocharacteristic bands in the ultra violet, a weakband at 275 Mµ with Mn++, a strong band at 247 mµ with Ni++. The presence of a band at 255 mµ in NaCl.Cu++ was confirmed. No evidence was obtained for the formation of Z-bands by Mn++, Ni++ or Cu++. Calcium was found to be much more soluble in NaCl than manganese, and enhanced the colourability proportionately less, pointing to a close relationship between solubility, mis-match, and the production of negative ion vacancies. Analysis of the F-band in coloured NaC1.Ca++ crystals indicated that Z- centres were formed by X-rays. Quenching enhanced the number of Z-centres, indicating a deposition at dislocations similar to that found for manganese. Evidence for the formation of both Z(_1)- and Z(_2)- centres by X-irradiation was found. In crystals containing a high proportion of Ca++ a band at 345 mp, which is probably to be associated with Ca++- positive ion vacancy complexes, was prominent. The results of an investigation of the effect of concentration of Ca++ on the growth of the F- and Z-bands indicated that a Ca++ ion introduced about ten negative ion vacancies into the crystal. On the basis of a statistical mechanical model of the formation of Z-centres it was inferred that of the Ca++ ions producing Z-centres no more than 2% were associated with positive ion vacancies, confirming the work of Etzel (l952). It was also inferred that the activation energy associated with the liberation of Ca++ ions from dislocations to form incipient Z-centres was 1.9 eV, a value which agreed well with an estimated value. Factors influencing the width of the F-band are also discussed.