The colour centre absorption of X‐Ray coloured alkaline earth fluoride crystals

Absorption by colour centres (i. e. the Ψ‐centre comprising an anion vacancy with a trapped electron) is observed in pure alkaline earth fluoride crystals after subtractive coloration at temperatures of 300, 84, and 37 °K. Colorability increases with decreasing temperature and increasing cation radius of the crystal hosts. The coloration spectra of the CaF 2 and SrF 2 crystals are similar; they consist of a strong double band, called the Ψ‐band, a weak ultraviolet band, probably caused by hole imperfections, called Ω‐centres, and an increase in absorption at 200 nm (Ω‐centre: collective term for all hole imperfections occuring in alkine earth fluoride crystals [8]). The peak of the Ψ‐band occurs at 375 nm for CaF 2 and at 434nm for SrF 2 . BaF 2 crystals exhibit two different coloration spectra depending on whether they are coloured at room temperature or at lower temperatures. Both the coloration spectra of BaF 2 are dissimilar to the type of spectrum observed for CaF 2 and SrF 2 . On heating the CaF 2 and SrF 2 crystals, strong discoloration is only observed at relatively high temperatures (between 125 and 150 °K); on the other hand, coloration increased when it is performed at 37 instead of 84 °K. Saturation of the coloration of the BaF 2 crystals takes place at 84 °K whereas their discoloration commences already between 85 and 100 °K. The discussion of the results considers special models and uses published results for additive coloration spectra, electron spin resonance and theoretical calculations.