Control of optical properties of YAG crystals by thermal annealing

•Optical properties of YAG crystals grown under different conditions are compared.•Carbon admixture with the concentration up to 0.01 wt% in YAGMo crystals grown under reducing conditions is found.•A procedure of YAGMo irreversible bleaching in the 200–1100 nm range by heat treatment is proposed. Optical properties of YAG crystals grown and annealed under different atmosphere conditions have been compared. Simultaneously we have registered the surface composition of crystals and content of basic admixtures in the crystals grown under the reducing conditions. Unlike YAG grown under weakly oxidizing conditions in Ir crucibles and bleached under oxidizing annealing, YAGMo crystals grown in Mo crucibles under reducing Ar + CO atmosphere can be bleached by both oxidizing and reducing thermal annealing. The bleaching of YAGMo is not reversed by further annealing under any available conditions. Mechanisms of this phenomenon have been discussed, including a possible role of admixtures in elimination of color centers in YAG grown under the reducing conditions.