Tungsten Bronze Field and Melt Growth of Crystals in the Na2O-BaO-Nb2O5 System

The extents of the liquidus and solidus fields were determined for tungsten bronze-type solid solutions in the Na₂O-BaO-Nb₂O₅ system by DTA and melt crystal growth experiments. Bronze-type solid solutions exist to 7.1Na₂O-34.9BaO-58Nb₂O₅ in the Nb₂O₅-rich region and from 12Na₂O-38BaO-50Nb₂O₅ to 4.6Na₂O-45.4BaO-50Nb₂O₅ along the NaNbO₂-BaNb₂O₆ join, which includes NaBa₂Nb₅O₁₅=10Na₂O-40BaO-50Nb₂O₆. There is little, if any, solid solubility of compositions with a deficiency of Nb₂O₅. Curie temperatures decline rapidly and dielectric constant peaks broaden with Nb₂O₅ substitution because the Nb:O ratio becomes greater than the octahedral 1:3 ratio. Useful ferroelectrics exist along the NaNbO₃-BaNb₂O₆ join where the Nb:O ratio is 1:3. Large striae-free crystals, with less optical scattering than Czochralski-grown crystals, were grown from unseeded Na₂O-rich melts (e.g. 15Na₂O-37.5BaO-47.5Nb₂O₅) cooled from 1520° to 1300°C at 2°C/h. Annealing effects on these crystals whose compositions lie on the NaNbO₃-BaNb₂O₆ join are discussed.