corundum, color.4, inclusions.5, treatments.6, gems

A time-dependent phase stability diagram has been developed describing the structural evolution of TiO₂ precipitates in Ti-doped sapphire (α-Al₂O₃). A high-pressure TiO₂ phase with the α-PbO₂ structure (α-TiO₂) forms as a coherent precipitate in Ti-doped sapphire annealed at 1200°–1300 °C; extended annealing times at these temperatures result in twinned particles, the twinning minimizing the precipitate–matrix misfit strain energy. Twinned α-TiO₂ eventually transforms to the rutile structure, coincident with the loss of coherency with the sapphire matrix. Rutile nucleates directly in sapphire annealed at temperatures at 1350 °C and above. Rutile is present mainly as semicoherent and twinned precipitates, the twinning occurring relatively early in the precipitation sequence. RWH: Great paper on the formation of rutile silk in corundum.