Oriented Nucleation of both Ge-Fresnoite and Benitoite/BaGe(4)O(9) during the Surface Crystallisation of Glass Studied by Electron Backscatter Diffraction

Two glasses of the compositions 2 BaO - TiO(2) - 2.75 GeO(2) and 2 BaO – TiO(2) –3.67 GeO(2) (also known as BTG55) are annealed at temperatures from 680 to 970 °C to induce surface crystallization. The resulting samples are analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) including electron backscatter diffraction (EBSD). Ge-Fresnoite (Ba(2)TiGe(2)O(8), BTG) is observed at the immediate surface of all samples and oriented nucleation is proven in both compositions. After a very fast kinetic selection, the crystal growth of BTG into the bulk occurs via highly oriented dendrites where the c-axes are oriented perpendicular to the surface. The growth of this oriented layer is finally blocked by dendritc BTG originating from bulk nucleation. The secondary phases BaTiGe(3)O(9) (benitoite) and BaGe(4)O(9) are also identified near the surface by XRD and localized by EBSD which additionally indicates orientation preferences for these phases. This behaviour is in contrast with previous reports from the Ba(2)TiSi(2)O(8) as well as the Sr(2)TiSi(2)O(8) systems.