PHASE-TRANSFORMATION OF TIO2 PRECIPITATES IN SAPPHIRE (ALPHA-AL2O3) INDUCED BY THE LOSS OF COHERENCY

The first phase to precipitate in Ti-doped sapphire (Ti:alpha-Al2O3) s ingle crystals annealed at 1300 degrees C in air is a high-pressure fo rm of TiO2 with the alpha-PbO2 structure (alpha-TiO2). The coherent pr ecipitates form as oblate spheroidal plates with habit planes parallel to (0001)(s). Because of a 5% lattice mismatch with sapphire, the sma ll coherent alpha-TiO2 precipitates are under an effective compressive stress, which stabilizes the high-pressure polymorph. During coarseni ng, the particles lose coherency by emitting a dislocation loop with a 1/3 <[1(1)over bar 01](s)> Burgers vector, which reduces the stress o n the particles and allows them to transform to the rutile structure. The orientation relationship between the sapphire matrix, alpha-TiO2 a nd rutile has been determined and a lattice correspondence for the pha se transformations established. Although the transformation from alpha -TiO2 to rutile is purely structural, its rate is controlled by atomic diffusion; the increased atomic volume of rutile compared with alpha- TiO2 is accommodated by expelling extra mass of Al2O3 into nearby inte rstitial dislocation loops and is associated with a change in the prec ipitate morphology.