A theory was presented which successfully explained the polarization, and the isothermal and isochronal behaviors, of the optical absorption bands which were responsible for the color of blue and blue-green sapphire (Fe,Ti-doped -Al2O3). The experimental data upon which the theory was based had conclusively shown that (Fe,Ti) and (Fe,Fe) vacancy-containing defect clusters were responsible for the optical properties of Fe,Ti-doped and H,Fe,Ti-doped materials. The experimental results also proved that VAl´´´ was the charge-compensating defect for TiAl• in Fe,Ti-doped and Ti-doped material, with a [TiAl•] = 3[VAl´´´] electroneutrality condition, and that VO•• provided the charge balance for FeAl´ in Fe-doped material; with a [FeAl´] = 2[VO••] electroneutrality condition. Defect clusters were found to form via diffusion-limited solid-state reactions in which the relative concentrations of charged and neutral point defects depended upon both the association energy and the diffusivity of the defects which participated in the clustering reaction.

A.R.Moon, M.R.Phillip: Journal of the American Ceramic Society, 1994, 77[2], 356-67