Bi1−xNbxO1.5+x (x=0.0625, 0.12) fast ion conductors: structures, stability and oxide ion migration pathways
No Thumbnail Available
Date
2015-05
Journal Title
Journal ISSN
Volume Title
Publisher
Elsevier
Abstract
A combined experimental and computational study of Bi1−xNbxO1.5+x (x=0.0625 and 0.12) has been carried out using laboratory X-ray, neutron and electron diffraction, impedance measurements and ab-initio molecular dynamics. We demonstrate that Bi0.9375Nb0.0625O1.5625, previously reported to adopt a cubic fluorite-type superstructure, can form two different polymorphs depending on the synthetic method: a metastable cubic phase is produced by quenching; while slower cooling yields a stable material with a tetragonal √2×√2×1 superstructure, which undergoes a reversible phase transition into the cubic form at ~680 °C on subsequent reheating. Neutron diffraction reveals that the tetragonal superstructure arises mainly from ordering in the oxygen sublattice, with Bi and Nb remaining disordered, although structured diffuse scattering observed in the electron diffraction patterns suggests a degree of short-range ordering. Both materials are oxide ion conductors. On thermal cycling, Bi0.88Nb0.12O1.62 exhibits a decrease in conductivity of approximately an order of magnitude due to partial transformation into the tetragonal phase, but still exhibits conductivity comparable to yttria-stabilised zirconia (YSZ). Ab-initio molecular dynamics simulations performed on Bi0.9375Nb0.0625O1.5625 show that oxide ion diffusion occurs by O2− jumps between edge- and corner-sharing OM4 groups (M=Bi, Nb) via tetrahedral □M4 and octahedral □M6 vacancies. © 2015 Elsevier Inc.
Description
Keywords
Oxides, Ions, X-ray diffraction, Neutron diffraction, Electron diffraction, Impedance, Phase transformations
Citation
Tate, M. L., Hack, J., Kuang, X., McIntyre, G. J., Withers, R. L., Johnson, M. R., Evans, I. R. (2015). Bi1−xNbxO1.5+x (x=0.0625, 0.12) fast ion conductors: structures, stability and oxide ion migration pathways. Journal of Solid State Chemistry, 225, 383-390. doi:10.1016/j.jssc.2015.01.006