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The colour centre absorption of X‐Ray coloured alkaline earth fluoride crystals
Author(s) -
Görlich P.,
Karras H.,
Symanowski Ch.,
Ullmann P.
Publication year - 1968
Publication title -
physica status solidi (b)
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.51
H-Index - 109
eISSN - 1521-3951
pISSN - 0370-1972
DOI - 10.1002/pssb.19680250107
Subject(s) - alkaline earth metal , absorption band , spectral line , absorption spectroscopy , crystal (programming language) , chemistry , absorption (acoustics) , fluoride , vacancy defect , ion , analytical chemistry (journal) , crystallography , alkali metal , optics , inorganic chemistry , physics , organic chemistry , chromatography , astronomy , computer science , programming language
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.