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

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), with a tetragonal tungsten bronze (TTB)-type structure (P4bm (No. 100); a=12.558(1), c=3.9708(3) Å), 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°C, approximately 15°C 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:7Nb2O5 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) Å) and an X-ray powder pattern analogous to that of hexagonal BaTa2O6. Also reported here are single-crystal X-ray structural refinements of Ba5Nb4O15 (P3m1 (No. 164); a=5.7960(5), c=11.7880(6) Å; Z=1) and air/water sensitive Ba3Nb2O8 (R3m (No. 166); a=6.0477(13), c=21.289(5) Å; 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 Ba5.75Al0.75Nb9.25O30, four phases with TTB-related superstructures in the BaO–Nb2O5 system (i.e. Ba3.3Nb10O28.3, Ba3Nb10O28, BaNb4O11, and BaNb6O16), and Ba0.9Nb2O5.9.