Additive coloration of sapphire

F-type centers have been introduced in to single crystals of α-Al2O3 by heating them at 2000 °C in a graphite crucible under strongly reducing conditions. This treatment produces results similar to additive coloration in that anion vacancies charge compensated by electrons are introduced as demonstrated by the presence of a 6.1-eV optical absorption band after heating. Although the same band can be created by high-energy-particle bombardment; by contrast those resulting from this additive coloration procedure are very stable against thermal annealing. Annealing above 1400 °C in air is necessary to produce a decrease in the 6.1-eV-band amplitude. The 6.1-eV band has been related to the F center in our previous study. This study further substantiates this model.