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. X-ray powder diffraction data confirmed that all Bi-Fe-Nb-O pyrochlores form with displacively disordered A2O networks. A structural refinement of the pyrochlore 0.4400:0.2700:0.2900 Bi2O3:Fe2O3:Nb2O5 using neutron powder diffraction data is reported. 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. Bi-Fe-Nb-O pyrochlores were readily obtained as single crystals and also as textured thin films used pulsed laser deposition.
Citation: Journal of Solid State Chemistry
Pub Type: Journals
Bi2O3-Fe2O3-Nb2O5, phase diagram, phase equilibria