Stability Phase-Fields and Pyrochlore Formation in Sections of the Bi2O3-Al2O3-Fe2O3-Nb2O5System

Bismuth-niobate-based ceramic materials are of interest for embedded elements such as capacitors and filters because they exhibit high relative dielectric permittivities and tend to be processible at temperatures in the 1000°–1200°C range. As part of ongoing studies of phase equilibria and structure–property relations in this class of electroceramics, a limited study of sections in the Bi2O3–Al2O3–FeO3–Nb2O5 system was conducted. In contrast to published reports, a pyrochlore phase was not found in the ternary subsystem Bi2O3–Al2O3–Nb2O5, despite detailed syntheses, which included controlled heatings up to and above the solidus: except for solid solutions of Al2O3 in binary Bi2O3–Nb2O5 phases (i.e., fluorite-related Bi3NbO7 and Bi5Nb3O15), no distinct ternary compounds were observed to form. In the quaternary Bi2O3–Al2O3–FeO3–Nb2O5 system, a pyrochlore solid solution was confirmed along a line from Bi1.657Fe1.092Nb1.150O7 (=42.50:28.00:29.50 Bi2O3:Fe2O3:Nb2O5) up to the limiting composition Bi1.657Al0.328Fe0.764Nb1.150O7 (=42.50:8.400:19.60:29.50 Bi2O3:Al2O3:Fe2O3:Nb2O5), corresponding to substitution of 30% of the Fe3+ with Al3+. This result suggests that the large pyrochlore phase-field in the Bi–Fe–Nb–O system extends into the quaternary Bi–Al–Fe–Nb–O system to form a single-phase volume of pyrochore solid solutions by dissolving appreciable amounts of Al2O3. The Bi–Al–Fe–Nb–O pyrochlore phases exhibited low-temperature dielectric relaxation characteristic of bismuth-based pyrochlores. The substitution of Al for Fe resulted in a small increase in relative dielectric permittivity (from 125 to ≈135 at 1 MHz, ≈200 K), and a decrease in the temperature (from ≈130 to ≈120 K) corresponding to the peak in the dielectric loss.