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Publication Citation: Phase Equilibria and Crystal Chemistry in the BaO-Al2O3-Nb2O5 and BaO-Nb2O5 Systems

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Author(s): Terrell A. Vanderah; B C. Collins; Winnie K. Wong-Ng; Robert S. Roth; L Farber;
Title: Phase Equilibria and Crystal Chemistry in the BaO-Al2O3-Nb2O5 and BaO-Nb2O5 Systems
Published: November 01, 2002
Abstract: Subsolidus phase equilibria in the BaO:Al2O3:Nb2O5 system at ~1250 C in air have been determined. Ternary compound formation in this system is limited to one new phase, Ba5.75Al0.75Nb9.25O30 (Ba6-xAl1-xNb9+xO30, x=0.25; P4bm (No. 100); a=12.558(1), c=3.9708(3) ), with a tetragonal tungsten bronze (TTB) type structure, and solid solutions of 1-2 mol% Al2O3 in TTB-related phases that form in the binary BaO-Nb2O5 system from 25 to 41 mol% BaO. Ba5.75Al0.75Nb9.25O30 exhibited an ambient dielectric constant of 242 according to capacitance measurements at 1 MHz; no indication of ferroelectric behavior was observed in the temperature range 100-400 K. This TTB-type phase exhibited no detectable range in composition and decomposed near 1275 C, approximately 15 C below the solidus. For the binary BaO-Nb2O5 phase diagram, the results of the present study suggest several alterations in the region between 0 and 50 mol% BaO. Single-crystal structural refinements were carried out for Ba5Nb4O15 and air/water sensitive Ba3Nb2O8. Indexed X-ray powder diffraction data are given for Ba5.75Al0.75Nb9.25O30, four phases with TTB-related superstructures in the BaO-Nb2O5 system (Ba3.3Nb10O28.3, Ba3Nb10O28, BaNb4O11, and BaNb6O16), and Ba0.9Nb2O5.9.
Citation: Journal of Alloys and Compounds
Volume: 346
Issue: No. 1-2
Keywords: Ba0.9Nb2O5.9;Ba3Nb2O5.9;Ba3Nb2O8;Ba5.75Al0.75Nb9.25O30;Ba5Nb4O15;BaO:Al2O3:Nb2Os;barium aluminum niobates;phase diagram;tetragonal tungsten bronze
Research Areas: Ceramics, Materials Science