Phase Equilibria in the System BaTiO3‐SiO2

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Abstract
The system BaTiO3–SiO2 is characterized by two congruently melting compounds and a compound having a subsolidus stability maximum. A barium analogue of the mineral sphene, BaTiSiO5, was shown to melt congruently at 1400°C, and BaTiSi2O7 melted congruently at 1250°C. The gem mineral benitoite (BaTiSi3O9) was synthesized hydrothermally and was shown to decompose below the solidus to form BaTi‐So2O7 and SiO2 when heated above 965°C, hydrothermally or above 1050°C, dry. Three eutectics in the system were determined: at 1245°C. and 70 mole % SiO2, at 1246°C. and 63% SiO2, and at 1260°C. and about 29% SiO2. Glasses with high refractive indices were obtained over a limited range of compositions. The cubic‐hexagonal transition temperature in barium titanate was raised sharply by small additions of silica. The maximum inversion temperature observed was 1575°C. and must have been due to limited solid solution of SiO2 in BaTiO3.Summary: Three intermediate compounds have been shown to exist in the system BaTiO3‐SiO2. The barium analogue of sphene, BaTiSiO5, melts congruently at 1400°C, and BaTiSi2O7 melts congruently at 1250°C. The compound BaTiSi3O9 (benitoite) was found to dissociate to form BaTiSi2O7 and SiO2 at 1050°C, under dry conditions and at 965°C, hydro‐thermally. This important gem mineral was synthesized from completely melted and quenched stoichiometric mixtures treated hydrothermally below 965°C, and at a water pressure of 5000 lb, per sq. in.There are three simple eutectics in the system: one between BaTiSi2O7 and SiO2 at 1245°C, and 70 mole % SiO2; one between BaTiSi2O7 and BaTiSiO5 at 1246°C, and 63% SiO2; and one between BaTiSiO5 and BaTiO3 at 1260°C, and about 29% SiO2.It was fairly easy to obtain glasses having refractive indices between 1.70 and 1.88 over a composition range of 50 to 75% Silica.The data pertaining to the cubic‐hexagonal transition in barium titanate were best explained on the basis of limited solid solution. As a result of the solid solution, the cubic‐hexagonal transition temperature is raised sharply from 1460°C. for the pure barium titanate to about 1575°C. by small additions of silica. The limits of solid solubility are approximately 8 and 4% SiO2 for the cubic and hexagonal forms, respectively.

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