Phase formation, crystal chemistry, and properties in the system Bi2O3–Fe2O3–Nb2O5

Subsolidus phase relations have been determined for the Bi2O3–Fe2O3–Nb2O5 system in air (900–1075 °C). Three new ternary phases were observed—Bi3Fe0.5Nb1.5O9 with an Aurivillius-type structure, and two phases with approximate stoichiometries Bi17Fe2Nb31O106 and Bi17Fe3Nb30O105 that appear to be structurally related to Bi8Nb18O57. The fourth ternary phase found in this system is pyrochlore (A2B2O6O′), which forms an extensive solid solution region at Bi-deficient stoichiometries (relative to Bi2FeNbO7) suggesting that ≈4–15% of the A-sites are occupied by Fe3+. X-ray powder diffraction data confirmed that all Bi–Fe–Nb–O pyrochlores form with positional displacements, as found for analogous pyrochlores with Zn, Mn, or Co instead of Fe. A structural refinement of the pyrochlore 0.4400:0.2700:0.2900 Bi2O3:Fe2O3:Nb2O5 using neutron powder diffraction data is reported with the A cations displaced (0.43 Å) to 96g sites and O′ displaced (0.29 Å) to 32e sites (Bi1.721Fe0.190(Fe0.866Nb1.134)O7, Fd3¯m (#227), a=10.501 (1) Å). This displacive model is somewhat different from that reported for Bi1.5Zn0.92Nb1.5O6.92, which exhibits twice the concentration of small B-type cations on the A-sites as the Fe system. Bi–Fe–Nb–O pyrochlores exhibited overall paramagnetic behavior with large negative Curie–Weiss temperature intercepts, slight superparamagnetic effects, and depressed observed moments compared to high-spin, spin-only values. The single-phase pyrochlore with composition Bi1.657Fe1.092Nb1.150O7 exhibited low-temperature dielectric relaxation similar to that observed for Bi1.5Zn0.92Nb1.5O6.92; at 1 MHz and 200 K the relative permittivity was 125, and above 350 K conductive effects were observed.