Colour changes of YAG crystals doped with rare earth ions and annealed in hydrogen, oxygen, vacuum or argon are described. The influence of UV irradiation is also given. Some crystals were further doped with Ca, Sr, Ba, or Cr, respectively. Yellow or brown colouring of the crystals is attributed to the O − centre formation which causes an increase of the absorption peak at 395–410 nm. Most intensive colouring showed the crystals containing a combination of Yb or Eu with Nd or Cr after oxygen annealing followed by UV irradiation.
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Kvapil et al. (1975) studied this question.
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