Phase formation, crystal chemistry, and properties in the system Bi 2O 3–Fe 2O 3–Nb 2O 5

Abstract

Subsolidus phase relations have been determined for the Bi 2O 3–Fe 2O 3–Nb 2O 5 system in air (900–1075 °C). Three new ternary phases were observed—Bi 3Fe 0.5Nb 1.5O 9 with an Aurivillius-type structure, and two phases with approximate stoichiometries Bi 17Fe 2Nb 31O 106 and Bi 17Fe 3Nb 30O 105 that appear to be structurally related to Bi 8Nb 18O 57. The fourth ternary phase found in this system is pyrochlore ( A 2 B 2O 6O′), which forms an extensive solid solution region at Bi-deficient stoichiometries (relative to Bi 2FeNbO 7) suggesting that ≈4–15% of the A-sites are occupied by Fe 3+. 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 Bi 2O 3:Fe 2O 3:Nb 2O 5 using neutron powder diffraction data is reported with the A cations displaced (0.43 Å) to 96 g sites and O′ displaced (0.29 Å) to 32 e sites (Bi 1.721Fe 0.190(Fe 0.866Nb 1.134)O 7, Fd3¯ m (#227), a = 10.508 ( 1 ) Å ). This displacive model is somewhat different from that reported for Bi 1.5Zn 0.92Nb 1.5O 6.92, 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 Bi 1.657Fe 1.092Nb 1.150O 7 exhibited low-temperature dielectric relaxation similar to that observed for Bi 1.5Zn 0.92Nb 1.5O 6.92; at 1 MHz and 200 K the relative permittivity was 125, and above 350 K conductive effects were observed.