Tungsten Bronze Field and Melt Growth of Crystals in the Na 2 O‐BaO‐Nb 2 O 5 System

The extents of the liquidus and solidus fields were determined for tungsten bronze‐type solid solutions in the Na 2 O‐BaO‐Nb 2 O 5 system by DTA and melt crystal growth experiments. Bronze‐type solid solutions exist to 7.1Na 2 O‐34.9BaO‐58Nb 2 O 5 in the Nb 2 O 5 ‐rich region and from 12Na 2 O‐38BaO‐50Nb 2 O 5 to 4.6Na 2 O‐45.4BaO‐50Nb 2 O 5 along the NaNbO 2 ‐BaNb 2 O 6 join, which includes NaBa 2 Nb 5 O 15 =10Na 2 O‐40BaO‐50Nb 2 O 6 . There is little, if any, solid solubility of compositions with a deficiency of Nb 2 O 5 . Curie temperatures decline rapidly and dielectric constant peaks broaden with Nb 2 O 5 substitution because the Nb:O ratio becomes greater than the octahedral 1:3 ratio. Useful ferroelectrics exist along the NaNbO 3 ‐BaNb 2 O 6 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 Na 2 O‐rich melts (e.g. 15Na 2 O‐37.5BaO‐47.5Nb 2 O 5 ) cooled from 1520° to 1300°C at 2°C/h. Annealing effects on these crystals whose compositions lie on the NaNbO 3 ‐BaNb 2 O 6 join are discussed.