Formation of biparticles in nominally undoped Y3Al5O12 crystals?

Nominally undoped YAG crystals become brown, when they are irradiated with hard rays at room temperature (RT). Investigations of absorption and fluorescence are discussed. Two fundamental types of coloration-independent fluorescence can be correlated with two fundamental types of coloration of nominally pure crystals of any origin. Mixed forms exist between both types. An experimentally found correspondence between both types of fluorescence suggests a mutual dependence, which can be understood by the formation of biparticles. Fe3 + and Mn4 + impurities could be shown to be at least promoting factors for the RT colourability of YAG crystals. Instead of Fe3 + might be assumed the presence of separated Mn2 + ions, too. Mn4 + would be charge-compensated then otherwise.