Oriented nucleation and crystal growth of Ge-fresnoite (Ba2TiGe2O8) in 2BaO·TiO2·2GeO2 glasses with additional GeO2

Abstract

Glasses of the mol composition 2BaO·TiO₂·2GeO₂ + xGeO₂ (with x = 0.0, 0.2, 0.75 and 1.5 mol) were prepared, polished and crystallised at 730 and 970 °C for 5 h. All glasses showed the surface crystallisation of Ge-fresnoite (Ba₂TiGe₂O₈) as well as the formation of BaGe₄O₉ at 970 °C. An additional unidentified phase occurs at the surfaces of the glass-ceramics with x = 0.0 and x = 0.2. The crystal orientations at the immediate surfaces were analysed by electron backscatter diffraction (EBSD) and showed that all glasses show a non-random orientation distribution, i.e. oriented nucleation at the surface. The intensity of the orientation preference increases with an increased amount of additional GeO₂ and crystallising at higher temperatures leads to more discrete orientation preferences. BaGe₄O₉ shows the same oriented nucleation as Ge-fresnoite. The crystal growth into the bulk was analysed by EBSD and generally shows a growth selection towards the 001-texture previously associated with dendritic growth in this system. Crystallising the stoichiometric glass at 730 °C, however, led to the formation of a 110-texture during growth into the bulk. All analysed glasses show bulk nucleation except for the glass with 1.5 mol additional GeO₂ where the opposing growth fronts collided in the middle of the ca. 1 mm thick sample. The effects of adding network formers to stoichiometric Ba-, Sr- and Ge-fresnoite glass-ceramics are compared and discussed.