CaF 2 and SrF 2 crystals are doped with trivalent yttrium ions, and X‐irradiated, which results in similar coloration spectra for both substances. The bands seem to satisfy MOLLWO 's relation v a 2 = const, since the SrF 2 bands are shifted to longer wavelengths compared with the corresponding CaF 2 bands. The absorption spectra is interpreted in terms of the 4 d electrons of the bivalent yttrium ions, whose formation is explained by the reductive action of the X‐rays. The coloration spectra of the CaF 2 /Y crystals are identical with the well‐known four‐absorption‐band spectra of natural crystals or synthetic crystals grown from natural material. This supports strongly the assumption that the four‐absorption‐band spectra in natural CaF 2 is produced by yttrium impurities; this however invalidates hypotheses according to which the four‐absorption‐band spectrum of CaF 2 , obtained after irradiation, is typical of intrinsic lattice imperfections in alkaline earth fluorides.
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Görlich et al. (1963) studied this question.
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