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
Issue: No. 1-2
Pub Type: Journals
Ba<sub>0.9</sub>Nb<sub>2</sub>O<sub>5.9</sub>, Ba<sub>3</sub>Nb<sub>2</sub>O<sub>5.9</sub>, Ba<sub>3</sub>Nb<sub>2</sub>O<sub>8</sub>, Ba<sub>5.75</sub>Al<sub>0.75</sub>Nb<sub>9.25</sub>O<sub>30</sub>, Ba<sub>5</sub>Nb<sub>4</sub>O<sub>15</sub>, BaO:Al<sub>2</sub>O<sub>3</sub>:Nb<sub>2</sub>Os, barium aluminum niobates, phase diagram, tetragonal tungsten bronze