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

Introduction of oxygen vacancies in alpha-Al2O3 has been quite challenging because Of its high thermal stability and high melting temperature. Here, we present it method to introduce oxygen vacancies in alpha-Al2O3. The method is based oil vacuum heating of a polycrystalline alpha-Al2O3 powder, producing a sphere-shaped solid consisting of alpha-Al2O3 single crystals. The thus prepared alpha-Al2O3 contains not only oxygen monovacancies, namely, the F+ and F centers, but also aggregated oxygen vacancies, including the F-2 and F-2(+) centers. We have found that a completely reversible interconversion between the F- and F-2-type centers occurs by irradiating different wavelengths of light. For example, similar to 90% of the F-2 and F-2(+) centers can be converted to each other by alternate irradiation of similar to 300 and similar to 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 F, and F centers by irradiation of similar to 300 and similar to 220 nm light, respectively, and the subsequent electron capture by the positively charged F+ and F-2(+) centers. In the present alpha-Al2O3 crystal, the positive charge of the F+ and F-2(+) centers is likely to be compensated by negatively charged aluminum vacancies, which can be introduced especially by the present vacuum heating process.