Effect of Reduction Annealing on the Coloration Mechanism of Yellow Sapphire with High Iron Content

The color of yellow sapphire from Africa characterized by high iron content and low levels of other transition metal elements was changed from yellow to grayish-blue after high-temperature reduction annealing. Before reduction annealing, the optical absorption spectra showed that the outer d-d electron transitions of Fe3+ were the main coloring cause of yellow sapphires, but the charge transfer between O2- and Fe3+ may have a greater contribution. The change in lattice parameter indicates that Fe3+ is reduced to Fe2+ during reduction annealing, and adjacent Fe2+ and Fe3+ form an Fe2+-Fe3+ ion pair. The absorption caused by intervalence charge transfer of Fe2+-Fe3+ is the essential reason for the grayish-blue appearance of yellow sapphires after reduction annealing. The charge compensation mechanism of Fe2+-Fe3+ in natural sapphire is also discussed, and oxygen vacancy is considered to be the most suitable charge compensator for Fe2+-Fe3+.