Subsolidus phase equilibria and properties in the system Bi2O3:Mn2O3±x:Nb2O5

Subsolidus phase relations have been determined for the Bi–Mn–Nb–O system in air (750–900 °C). Phases containing Mn2+, Mn3+, and Mn4+ were all observed. Ternary compound formation was limited to pyrochlore (A2B2O6O′), which formed a substantial solid solution region at Bi-deficient stoichiometries (relative to Bi2(Mn,Nb)2O7) suggesting that ≈14–30% of the A-sites are occupied by Mn (likely Mn2+). X-ray powder diffraction data confirmed that all Bi–Mn–Nb–O pyrochlores form with structural displacements, as found for the analogous pyrochlores with Mn replaced by Zn, Fe, or Co. A structural refinement of the pyrochlore 0.4000:0.3000:0.3000 Bi2O3:Mn2O3±x:Nb2O5 using neutron powder diffraction data is reported with the A and O′ atoms displaced (0.36 and 0.33 Å, respectively) from ideal positions to 96g sites, and with Mn2+ on A-sites and Mn3+ on B-sites (Bi1.6Mn2+0.4(Mn3+0.8Nb1.2)O7, image (♯227), image Å); evidence of A or O′ vacancies was not found. The displacive disorder is crystallographically analogous to that reported for Bi1.5Zn0.92Nb1.5O6.92, which has a similar concentration of small B-type ions on the A-sites. EELS spectra for this pyrochlore were consistent with an Mn oxidation between 2+ and 3+. Bi–Mn–Nb–O pyrochlores exhibited overall paramagnetic behavior with negative Curie–Weiss temperature intercepts, slight superparamagnetic effects, and depressed observed moments compared to high-spin, spin-only values. At 300 K and 1 MHz the relative dielectric permittivity of Bi1.600Mn1.200Nb1.200O7 was ≈128 with tan image; however, at lower frequencies the sample was conductive which is consistent with the presence of mixed-valent Mn. Low-temperature dielectric relaxation such as that observed for Bi1.5Zn0.92Nb1.5O6.92 and other bismuth-based pyrochlores was not observed. Bi–Mn–Nb–O pyrochlores were readily obtained as single crystals and also as textured thin films using pulsed laser deposition.