Oxygen vacancies and their aggregates were produced much more readily in Mg-doped α-Al2O3 than in undoped α-Al2O3 single crystals during thermochemical reduction at high temperatures. A reversible photochromic effect was discovered in Mg-doped Al2O3 crystals containing large concentrations of O divacancies. Alternate excitation with 5.0 and 3.69eV light resulted in brown or yellowish–green coloration, respectively. The yellowish–green coloration could also be restored by thermal anneals at about 750K.

Photochromism of Vacancy-Related Defects in Thermochemically Reduced α-Al2O3:Mg Single Crystals. R.Ramírez, M.Tardío, R.González, Y.Chen, M.R.Kokta: Applied Physical Letters, 2005, 86[8], 081914 (3pp)