Orange luminescence of α‐Al₂O₃ related to clusters consisting of F centers and divalent cations

The orange luminescence of α‐Al 2 O 3 under UV excitation is characterized by a 2.07‐eV orange broadband emission that has not yet been elucidated. This emission is present in natural and synthetic crystals and powders, as well as in Be‐treated samples. All orange‐luminescent materials have low Fe concentration (mostly <1000 ppm) with traces of divalent cations, mostly Mg, or Be in Be‐diffused material (dozens of ppm). Mg 2+ , Mn 2+ , and Be 2+ cations substitute for trivalent Al. To accommodate the charge deficit, several defects are created, including oxygen vacancies also called F centers. Indeed, our excitation spectra revealed the presence of several different F centers (F, F + , and clustered F 2 , F 2 + , F 2 2+ ) in those samples. However, the thermal stability and the measured luminescence lifetimes do not match with previously reported characteristics of isolated F centers. Based on our experiments, we suggest that a complex aggregate of two F centers (F 2 2+ ) trapped at divalent cations is a major cause of this uncommon microsecond lifetime emission, even if a variety of other defects, including Cr 3+ , V 3+ , or interstitial Al 3+ , are present.