Phase equilibria, crystal growth and characterization of the novel ferroelectric tungsten bronzes CaxBa1-xNb2O6 (CBN) and CaxSryBa1-x-yNb2O6 (CSBN)

Phase diagram analyses and crystal growth experiments of the novel ferroelectric tetragonal tungsten bronzes CaxBa1-xNb2O6 (calcium barium niobate, CBN) and CaxSryBa1-x-yNb2O6 (calcium strontium barium niobate, CSBN) were performed in this study. The quasi-binary phase diagram BaNb2O6-CaNb2O6 Was found to be similar to the related system BaNb2O6-SrNb2O6, but with a remarkably reduced existence region. In the quasi-ternary phase diagram CaNb2O6-SrNb2O6-BaNb2O6 the stability field of CaxSryBa1-x-yNb2O6 mixed crystals was determined by X-ray powder diffraction of sintered and annealed samples. Large, transparent, and nearly colorless CBN and CSBN single crystals of high quality were grown by the Czochralski method. The dislocation network structure of the core region in the as-grown crystals was analyzed by X-ray topography employing white beam synchrotron radiation. Precise measurements of the refractive index dispersion showed a composition dependance only for the extraordinary beam. The Curie temperature of the ferroelectric phase transition 4/m -> 4mm determinated by calorimetric measurements exhibited a clear composition dependance. It is expected that in the complete mixed crystal system CaxSryBa1-x-yNb2O6 the Curie temperature could be varied between about 80 degrees C for strontium barium niobate with x(sr) = 0.61 and about 300 degrees C for CBN with x(Ca) approximate to 0.20, the lowest possible Ca content. (C) 2007 Elsevier B.V. All rights reserved.