Oriented Nucleation and Crystal Growth of Ba-Fresnoite (Ba₂TiSi₂O₈) in 2 BaO·TiO₂·2 SiO₂ Glasses with Additional SiO₂

Glasses of the mol composition 2 BaO·TiO2·2 SiO2 + x SiO2 (with x = 0.0, 0.1, 0.2, 0.5, 0.75, 1.5, 2.0, and 3.0 mol) were prepared, polished, and crystallized at 810 °C for 5 h or more. All glasses showed the sole crystallization of Ba-fresnoite (Ba2TiSi2O8). The crystal orientations at the immediate surfaces were analyzed by EBSD and showed that all glasses exhibit a nonrandom orientation distribution, i.e., oriented nucleation at the surface. While the glasses with x = 0.0, 0.1, and 0.2 show rather broad orientation preferences in the form of c-axes tilted from the surface by roughly 10–50°, the glasses with x = 1.5 or higher show discrete textures where specific crystal orientations are significantly preferred with orientation spreads of less than ±15°. While the glasses with x = 0 and 0.1 show polygon crystals at the surface and in the bulk, dendritic crystals are observed in the bulk of all other analyzed compositions. Phase separation solely occurred in the glass with x = 1.5 during the applied heat treatments.