Defect Clustering and Color in Fe,Ti: α‐Al ₂ O ₃

A comprehensive theory is presented which successfully explains the polarization, isothermal, and isochronal behavior of the optical absorption bands responsible for the color in blue and blue‐green sapphire (Fe,Ti: α‐Al 2 O 3 ). The experimental study on which this theory is based has conclusively shown that (Fe,Ti) and (Fe,Fe) vacancy‐containing defect clusters are responsible for the optical properties of Fe,Ti: α‐Al 2 O 3 and H,Fe,Ti: α‐Al 2 O 3 . The experimental results also prove that V‴ is the charge‐compensating defect for Ti˙ Al in Fe, Ti and Ti: α‐Al 2 O 3 with an [Ti˙ Al ] = 3[V‴] electroneutrality condition and that V¨ o provides the charge balance for Fe′ Al in Fe: α‐Al 2 O 3 with an [Fe′ Al ] = 2[V¨ o ]electroneutrality condition. Defect clusters were found to form via diffusion‐limited solid‐state reactions where the relative concentration of charged and neutral point defects depends on both the association energy and the diffusivity of the defects participating in the clustering reactions. In this paper, a model is presented which attempts to explain both the origin and the thermal behavior of the optical bands responsible for the color of Fe,Ti: α‐Al 2 O 3 .