Phase equilibria and crystal chemistry in the BaO-Al2O3-Nb2O5 and BaO-Nb2O5 systems

Subsolidus phase equilibria in the BaO-Al2O3-Nb2O5 system at approximate to1250 degreesC 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), with a tetragonal tungsten bronze (TTB)-type structure (P4bm (No. 100); a = 12.558(1), c = 3.9708(3) Angstrom), 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 permittivity of 242 (1 MHz); no indication of ferroelectric behavior was observed in the temperature range 100-400 K. This TTB-type compound exhibited no detectable range in composition and decomposed to other solid phases near 1275 degreesC, approximately 15 degreesC below the solidus. For the binary BaO-Nb2O5 phase diagram, the results of the present study suggest the following alterations in the region between 0 and 50 mol% BaO: the high-BaO end of the TTB-related solid solution was observed to extend past the previously reported limit of Ba3Nb10O28 to 41 mol% BaO, and the compound previously reported to occur near 6BaO:7Nb(2)O(5) formed as a line compound at 47.5 mol% BaO (= Ba0.9Nb2O5.9), with a hexagonal unit cell (P6/mmm (No. 191); a = 21.044(1), c = 3.9787(2) Angstrom) and an X-ray powder pattern analogous to that of hexagonal BaTa2O6. Also reported here are single-crystal X-ray structural refinements of Ba5Nb4O15 (P (3) over bar m1 (No. 164); a = 5.7960(5), c = 11.7880(6) Angstrom; Z = 1) and air/water sensitive Ba3Nb2O8 (R (3) over barm (No. 166); a = 6.0477(13), c = 21.289(5) Angstrom; Z = 3). The results are in good agreement with previously suggested structural models from X-ray powder diffraction studies. Indexed X-ray powder diffraction data are given for Ba(5.75)Al(0.75)Nbg(9.25)O(30), four phases with TTB-related superstructures in the BaO-Nb2O5 system (i.e. Ba3.3Nb10O28.3, Ba3Nb10O28, BaNb4O11, and BaNb6O16), and Ba0.9Nb2O5.9. Published by Elsevier Science B.V.