Reversible Photoinduced Interconversion of Color Centers in α-Al₂O₃ Prepared under Vacuum

Abstract

Introduction of oxygen vacancies in α-Al2O3 has been quite challenging because of its high thermal stability and high melting temperature. Here, we present a method to introduce oxygen vacancies in α-Al2O3. The method is based on vacuum heating of a polycrystalline α-Al2O3 powder, producing a sphere-shaped solid consisting of α-Al2O3 single crystals. The thus prepared α-Al2O3 contains not only oxygen monovacancies, namely, the F+ and F centers, but also aggregated oxygen vacancies, including the F2 and F2+ centers. We have found that a completely reversible interconversion between the F- and F2-type centers occurs by irradiating different wavelengths of light. For example, ∼90% of the F2 and F2+ centers can be converted to each other by alternate irradiation of ∼300 and ∼220 nm light even at room temperature. The observed interconversion can be interpreted in terms of the photoionization of the neutrally charged centers such as F2 and F centers by irradiation of ∼300 and ∼220 nm light, respectively, and the subsequent electron capture by the positively charged F+ and F2+ centers. In the present α-Al2O3 crystal, the positive charge of the F+ and F2+ centers is likely to be compensated by negatively charged aluminum vacancies, which can be introduced especially by the present vacuum heating process.