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Subsolidus Phase Equilibria and Properties in the Magnetic Dielectric System Bi2O3:Mn2O3+x:Nb2O5

Published

Author(s)

Terrell A. Vanderah, M W. Lufaso, A U. Adler, Igor Levin, Juan C. Nino, Virgil Provenzano, Peter K. Schenck, Robert S. Roth

Abstract

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, which formed a substantial solid solution region at Bi-deficient stoichiometries. X-ray powder diffraction data confirmed that all Bi-Mn-Nb-O pyrochlores form with displacively disordered A2O networks. A structural refinement of the pyrochlore 0.4000:0.3000:0.3000 Bi2O3:Mn2O3 x:Nb2O5 using neutron powder diffraction data is reported with A and O atoms displaced (0.36 and 0.33 , 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, Fdm (#227), a = 10.478(1) ). The displacive disorder is crystallographically analogous to that reported for Bi1.5Zn0.92Nb1.5O6.92. 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. At 300 K and 1 MHz the relative dielectric permittivity of Bi1.600Mn1.200Nb1.200O7 was 128 with tan d = 0.05; however, at lower frequencies the sample was conductive which is consistent with the presence of mixed-valent Mn. Bi-Mn-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
Volume
179
Issue
11

Keywords

Bi-Mn-Nb-O, bismuth manganese niobates, dielectric properties, displacive disorder, phase diagram, phase equilibria, pyrochlore

Citation

Vanderah, T. , Lufaso, M. , Adler, A. , Levin, I. , Nino, J. , Provenzano, V. , Schenck, P. and Roth, R. (2006), Subsolidus Phase Equilibria and Properties in the Magnetic Dielectric System Bi<sub>2</sub>O<sub>3</sub>:Mn<sub>2</sub>O<sub>3+x</sub>:Nb<sub>2</sub>O<sub>5</sub>, Journal of Solid State Chemistry (Accessed February 27, 2024)
Created July 20, 2006, Updated February 19, 2017