Phase transformation of TiO₂ precipitates in sapphire (α-Al₂O₃) induced by the loss of coherency

Abstract

The first phase to precipitate in Ti-doped sapphire (Ti: α-Al₂O₃) single crystals annealed at 1300°C in air is a high-pressure form of TiO₂ with the α-PbO₂ structure (α-TiO₂). The coherent precipitates form as oblate spheroidal plates with habit planes parallel to (0001)_s. Because of a 5% lattice mismatch with sapphire, the small coherent α-TiO₂ precipitates are under an effective compressive stress, which stabilizes the high-pressure polymorph. During coarsening, the particles lose coherency by emitting a dislocation loop with a ⅓〈1101〉_s Burgers vector, which reduces the stress on the particles and allows them to transform to the rutile structure.

The orientation relationship between the sapphire matrix, α-TiO₂ and rutile has been determined and a lattice correspondence for the phase transformations established. Although the transformation from α-TiO₂ to rutile is purely structural, its rate is controlled by atomic diffusion; the increased atomic volume of rutile compared with α-TiO₂ is accommodated by expelling extra mass of Al₂O₃ into nearby interstitial dislocation loops and is associated with a change in the precipitate morphology.