Subsolidus phase equilibria and properties in the system Bi2O3:Mn2O3±x:Nb2O5
| Autor: | Virgil Provenzano, Juan C. Nino, Terrell A. Vanderah, A U. Adler, Igor Levin, Michael W. Lufaso, Peter K. Schenck |
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| Rok vydání: | 2006 |
| Předmět: |
Materials science
Neutron diffraction Pyrochlore engineering.material Condensed Matter Physics Electronic Optical and Magnetic Materials Inorganic Chemistry Crystallography Paramagnetism Nuclear magnetic resonance X-ray crystallography Materials Chemistry Ceramics and Composites engineering Physical and Theoretical Chemistry Stoichiometry Powder diffraction Phase diagram Solid solution |
| Zdroj: | Journal of Solid State Chemistry. 179:3467-3477 |
| ISSN: | 0022-4596 |
| Popis: | Subsolidus phase relations have been determined for the Bi-Mn-Nb-O system in air (750-900deg. C). Phases containing Mn{sup 2+}, Mn{sup 3+}, and Mn{sup 4+} were all observed. Ternary compound formation was limited to pyrochlore (A{sub 2}B{sub 2}O{sub 6}O'), which formed a substantial solid solution region at Bi-deficient stoichiometries (relative to Bi{sub 2}(Mn,Nb){sub 2}O{sub 7}) suggesting that {approx}14-30% of the A-sites are occupied by Mn (likely Mn{sup 2+}). 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 Bi{sub 2}O{sub 3}:Mn{sub 2}O{sub 3+}/-{sub x}:Nb{sub 2}O{sub 5} using neutron powder diffraction data is reported with the A and O' atoms displaced (0.36 and 0.33A, respectively) from ideal positions to 96g sites, and with Mn{sup 2+} on A-sites and Mn{sup 3+} on B-sites (Bi{sub 1.6}Mn{sup 2+}{sub 0.4}(Mn{sup 3+}{sub 0.8}Nb{sub 1.2})O{sub 7}, Fd3-bar m (-bar 227), a=10.478(1)A); evidence of A or O' vacancies was not found. The displacive disorder is crystallographically analogous to that reported for Bi{sub 1.5}Zn{sub 0.92}Nb{sub 1.5}O{sub 6.92}, which has a similar concentration of small B-type ions on the A-sites. EELS spectra for this pyrochlore were consistent withmore » 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 300K and 1MHz the relative dielectric permittivity of Bi{sub 1.600}Mn{sub 1.200}Nb{sub 1.200}O{sub 7} was {approx}128 with tan {delta}=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 Bi{sub 1.5}Zn{sub 0.92}Nb{sub 1.5}O{sub 6.92} 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.« less |
| Databáze: | OpenAIRE |
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