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

Published

November 1, 2002

Author(s)

Terrell A. Vanderah, B C. Collins, Winnie K. Wong-Ng, Robert S. Roth, L Farber

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, Ba_5.75Al_0.75Nb_9.25O₃₀ (Ba_6-xAl_1-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. Ba_5.75Al_0.75Nb_9.25O₃₀ 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 Ba_5Nb₄O₁₅ and air/water sensitive Ba₃Nb₂O₈. Indexed X-ray powder diffraction data are given for Ba_5.75Al_0.75Nb_9.25O₃₀, four phases with TTB-related superstructures in the BaO-Nb₂O₅ system (Ba_3.3Nb₁₀O_28.3, Ba₃Nb₁₀O₂₈, BaNb₄O₁₁, and BaNb₆O₁₆), and Ba_0.9Nb₂O_5.9.

Citation

Journal of Alloys and Compounds

Volume

346

Issue

No. 1-2

Pub Type

Journals

Keywords

Ba_0.9Nb₂O_5.9, Ba₃Nb₂O_5.9, Ba₃Nb₂O₈, Ba_5.75Al_0.75Nb_9.25O₃₀, Ba₅Nb₄O₁₅, BaO:Al₂O₃:Nb₂Os, barium aluminum niobates, phase diagram, tetragonal tungsten bronze

Citation

Vanderah, T. , Collins, B. , Wong-Ng, W. , Roth, R. and Farber, L. (2002), Phase Equilibria and Crystal Chemistry in the BaO-Al₂O₃-Nb₂O₅ and BaO-Nb₂O₅ Systems, Journal of Alloys and Compounds (Accessed December 11, 2024)