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, Fd3¯m (♯227), a=10.478(1)a=10.478(1) 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 δ=0.05δ=0.05; 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.

Graphical abstractTernary compound formation in the Bi–Mn–Nb–O system is limited to a pyrochlore solid solution which occurs at “Bi-deficient” stoichiometries compared to conventional A2B2O7-type formulations with Bi on the A-sites and Mn/Nb on the B-sites—all pyrochlores in this system exhibit displacive structural behavior.Figure optionsDownload full-size imageDownload as PowerPoint slide