Impurity‐Associated Colour Centres in Mg‐ and Ca‐Doped Al2O3 Single Crystals

In Mg‐ and Ca‐doped α‐Al2O3 crystals irradiated by X‐rays at 80 K new paramagnetic colour centres are observed having an absorption band at about 255 nm and an emission band at 310 nm. The hyperfine structure of the EPR spectrum indicates that the unpaired electron spin interacts mainly with two Al27 nuclei. The optical and EPR spectra are consistent with the model of a centre involving one electron trapped at an anion vacancy adjoining the substitutional divalent ion (FMg or FCa). The luminescence band at 290 nm observed in the TSL peak at 225 K is suggested to result from the recombination of the holes with the FMg(FCa) centres.