Phase transformation of TiO2precipitates in sapphire (α-Al2O3) induced by the loss of coherency

Abstract

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.