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

The orange luminescence of α-Al₂O₃ 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). Mg²⁺, Mn²⁺, and Be²⁺ 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 F₂, F₂ ⁺, F₂ 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 (F₂ ²⁺) trapped at divalent cations is a major cause of this uncommon microsecond lifetime emission, even if a variety of other defects, including Cr³⁺, V³⁺, or interstitial Al³⁺, are present.