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

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Abstract
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.