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A reversible photochromic effect was investigated in Mg-doped α-Al2O3 single crystals oxidized at elevated temperatures. Alternate illumination with blue and UV light at T⩽210 K results in reversible disappearance and reappearance of a gray-purple coloration, respectively. The coloration can also be fully rejuvenated by warming the crystal to T⩾215 K.
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Tardı́o et al. (2003) studied this question.
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