Oriented nucleation and crystal growth of Sr-fresnoite (Sr2TiSi2O8) in 2SrO·TiO2·2SiO2 glasses with additional SiO2

Abstract

Glasses of the mol composition 2SrO·TiO₂·2SiO₂ + xSiO₂ (with x = 0.0, 0.2, 0.75 and 1.5 mol) were prepared. They were polished and then heat treated for 5 h or more at 830 and 970 °C during which Sr-fresnoite (Sr₂TiSi₂O₈) solely crystallised in all glasses. EBSD was used to show that the crystal orientations at the immediate surfaces are non-random, meaning that the surface nuclei are already oriented. The intensity of the orientation preference is relatively low and an influence of the crystallisation temperature could not be determined. The crystal growth into the bulk was analysed by EBSD and showed that all glasses crystallise via a mechanism proposed to be viscous fingering when crystallised at 970 °C. The stoichiometric glass also shows this growth when crystallised at 830 °C whereas the glasses containing additional SiO₂ show a polygon growth selected towards an 001-texture when crystallised at this temperature. In contrast to the widespread lack of bulk nucleation in glasses of the SrO·TiO₂·2SiO₂ system, bulk nucleation occurs in the glasses containing 0 and 0.2 mol additional SiO₂.