New titanium oxide precipitates in Ti-doped sapphire

Abstract

When Ti-doped α-Al2O3 single crystals are annealed at 1400°C, needle-like rutile TiO2 precipitates form on the basal plane of α-Al2O3 and cause the asterism in star sapphire. Tetragonal rutile has a = 0.459 nm and c = 0.296 nm, and has the following orientation relationship with the α-Al2O3 matrix: (100)r // (0001)s and <011>r // <100>s, where the subscripts r and s refer to the rutile and sapphire, respectively. Moon and Phillips studied the precipitation in natural blue sapphire containing both Fe and Ti. They found that rutile precipitates formed after annealing at 1350°C but an orthorhombic α-TiO2 precipitate formed after annealing at 1150°C. In this study, Ti-doped α-Al2O3 single crystals were annealed at 1300°C. TEM specimens were prepared with their plane normals parallel to <110>s, <100>s and <0001>s, respectively, and ion beam thinned to electron transparency.Three different types of precipitates are present in the α-Al2O3 matrix. The first is the needlelike rutile precipitate lying on (0001)s, with the needle axes parallel to <100>s.