Study on low-temperature heat treatment of purplish red sapphire

A light purplish red sapphire is heat treated in an airtight crucible. The sample changes little in color after receiving heat treatment at 1100°C, but turns to light blue and blue after being treated at 1200°C and 1300°C, respectively. Before heating, the UV-VIS absorption spectra of the sample are dominated by the 551 nm broad absorption band contributed by the d-electron transition A24→T24 of Cr3+. After heating, the UV-VIS absorption spectra are dominated by the 563 nm broad absorption band contributed by the intervalence charge transfer of Fe2+−Ti4+. The x ray photoelectron spectroscopy test reveals that the Fe2+ and Ti4+ ion contents increase with increasing temperature. The sapphire changing from light purplish red to blue in the heating process is owing to the fact that the Fe2+ and Ti4+ contents grow and the intervalence charge transfer of Fe2+−Ti4+ selectively absorbs UV-VIS light.