The colour changes of YAG crystals doped with Ti, V, Cr, Mn, Fe, Co and Ni as well as those doped with Mg or Be are described. The comparison of samples annealed in hydrogen oxygen and argon and exposed to an UV irradiation is given. Yellow or brown colour of YAG crystals is attributed to the O− centre formation which causes an increase of absorption, particularly near 400 nm. Most intensive coloration showed the crystals containing iron group ions in a higher valency state after UV irradiation and some crystals (e. g. YAG:Cr) annealed in argon.
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Kvapil et al. (1975) studied this question.