Abstract
Two glasses of the compositions 2 BaO - TiO2 - 2.75 GeO2 and 2 BaO – TiO2 –3.67 GeO2 (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 (Ba2TiGe2O8, 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 BaTiGe3O9 (benitoite) and BaGe4O9 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 Ba2TiSi2O8 as well as the Sr2TiSi2O8 systems.
Highlights
Fresnoite in its original form has the composition Ba2TiSi2O8 (BTS)
While only BTG was detected after crystallizing a stoichiometric melt[17], phases of the compositions BaTiGe3O9 and BaGe4O9 have been reported at the immediate surface after crystallizing glasses containing additional GeO214,16,17
As the extinction coefficients of Ti3+ are very high and a violet coloration is not observed, it is unlikely that there is enough Ti3+ in the glass to show a measurable effect on the nucleation and crystallization behavior
Summary
Fresnoite in its original form has the composition Ba2TiSi2O8 (BTS). Barium may be partially or completely substituted by Sr to form Sr2TiSi2O8 (STS) while Si may partially or completely be replaced by Ge to form Ba2TiGe2O8 (BTG) or a solid solution. While only BTG was detected after crystallizing a stoichiometric melt[17], phases of the compositions BaTiGe3O9 (benitoite) and BaGe4O9 have been reported at the immediate surface after crystallizing glasses containing additional GeO214,16,17.
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