Type II Bi1-xWxO1.5+1.5x: a (3 + 3) – dimensional commensurate modulation that stabilises the fast ion conducting delta phase of bismuth oxide
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Date
2016-11-29
Journal Title
Journal ISSN
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Publisher
Australian Institute of Nuclear Science and Engineering
Abstract
The high temperature cubic polymorph of bismuth oxide, δ-Bi2O3, is the best intermediate temperature oxide ionic conductor known. Unfortunately, δ-Bi2O3 is only stable from 750-830°C, limiting its use as an ionic conductor. In order to stabilise its average fluorite-type structure to room temperature, while preserving a large part of its conductivity, higher valent transition metals such as Nb5+, Ta5+, Cr6+, Mo6+ or W6+ can be introduced, resulting in a variety of complex modulated structures based on the fluorite-type subcell.
We have identified a new member of this class of (3+3)-dimensional modulated phases in the Bi1-xWxO1.5+1.5x system, in which the modulation vector ε ‘locks in’ to a commensurate value of 1/3 [2]. The
structure was refined in a 3x3x3 supercell against single-crystal Laue neutron diffraction data. Detailed ab initio calculations were used to test and optimise the local structure of the oxygen sublattice around a single mixed Bi/W site. The underlying crystal chemistry was shown to be based on a transition from fluorite-type to pyrochlore-type via the formation of W4O18 ‘tetrahedra of octahedra’. The full range of occupancies on this mixed Bi/W site give a solid-solution range bounded by Bi23W4O46.5 (x = 0.148) and Bi22W5O48 (x = 0.185).
AC impedance measurements show promising results with ionic conductivities comparable to yttria stabilized zirconia. Ab initio molecular dynamics simulations combined with quasi-elastic (QENS) and inelastic neutron scattering (INS) experiments give first insights into the dynamics of the conduction process and diffusion mechanisms in these materials.
Description
Keywords
Temperature range 0400-1000 K, Bismuth oxides, Ionic conductivity, Fluorite, Transition elements, Monocrystals, Neutron diffraction, Pyrochlore, Impedance
Citation
Wind, J., Auckett, J. E., Withers, R. L., Piltz, R. O., Mole, R., & Ling, C. D. (2016). Type II Bi1-xWxO1.5+1.5x: a (3 + 3) – dimensional commensurate modulation that stabilises the fast ion conducting delta phase of bismuth oxide. Paper presented at 13th AINSE-ANBUG Neutron Scattering Symposium, Sydney, NSW, Australia, 29-30 November 2016.