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

Published

July 20, 2006

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

Pub Type

Journals

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 Bi₂O₃:Mn₂O_3+x:Nb₂O₅, Journal of Solid State Chemistry (Accessed April 24, 2024)