Browsing by Author "Wind, J"

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    Hydration mechanisms and proton conduction in the mixed ionic–electronic conductors Ba4Nb2O9 and Ba4Ta2O9
    (American Chemical Society, 2018-07-12) Wind, J; Mole, RA; Yu, DH; Avdeev, M; Ling, CD
    We studied the behavior of hydrogen in the mixed ionic–electronic conductors γ-Ba4Nb2O9 and 6H-Ba4Ta2O9 using a combination of experimental (neutron diffraction and inelastic neutron scattering) and computational (ab initio molecular dynamics) methods. Although these compounds have isostructural low-temperature polymorphs, they adopt distinct forms in the high-temperature conducting regime. We show here that they also have distinct mechanisms for hydration and ionic conduction. Hydration of γ-Ba4Nb2O9 is localized to 2-D layers in the structure that contain a 1:1 ratio of isolated but adjacent NbO4 and NbO5 polyhedra. OH– and H+ ions combine with two polyhedra, respectively, to form complete layers of NbO4OH polyhedra, giving rise to a stoichiometric hydrated form γ-III-Ba4Nb2O9·1/3H2O. Protons then diffuse through these 2-D layers by “hopping” between oxygen atoms on adjacent polyhedra. In the case of 6H-Ba4Ta2O9, hydration occurs by intercalating intact water molecules into the structure up to a maximum of ∼0.375 H2O per formula unit. This explains the unusual local and long-range structural distortions in the hydrated form observed by neutron diffraction. Diffusion then occurs by water molecules moving between neighboring symmetry equivalent positions. These fundamentally different hydration and proton conduction mechanisms explain why 6H-Ba4Ta2O9 has the less well-defined and higher maximum water content, while γ-Ba4Nb2O9 has the higher proton conductivity. © 2018 American Chemical Society
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    Inelastic neutron scattering studies of ?-Bi2O3 -related oxide-ion conductors
    (International Conference on Neutron Scattering, 2017-07-12) Ling, CD; Wind, J; Mole, RA
    Inelastic neutron scattering is the only experimental technique that simultaneously probes ionic diffusion (as quasielastic neutron scattering, QENS) and lattice dynamics (as a generalised density of states, GDOS). In solid-state ionic conductors where the diffusing species has a predominantly incoherent neutron scattering cross section – i.e., H – key parameters describing diffusion can be extracted directly by modelling the form of the QENS. QENS analysis is a far more complex (and unresolved) problem when the diffusing atoms have significant coherent cross-sections, such as O and Li. However, it is possible to interpret QENS indirectly in such cases by using the experimental GDOS to validate ab initio dynamics simulations, then interrogating the simulations to identify diffusion mechanisms. This is obviously most effective when the experimental signal is strong, i.e., conductivity is high. In the case of oxide-ionic conductors, this points to stabilised versionsof ?-Bi2O3. Here, we present new results for a series of cubic ?-Bi2O3-related compounds with (3+3)-D incommensurately modulated structures. We identify a mechanism by which oxide ions migrate through continuous and nearly isotropic channels. The results are compared to conductivity data and theoretical models for pure ?-Bi2O3, and used to suggest chemical modifications that could maximise performance and cycling stability.
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    Liquid-like ionic diffusion in solid bismuth oxide revealed by coherent quasielastic neutron scattering
    (American Chemical Society (ACS), 2017-08-07) Wind, J; Mole, RA; Yu, D; Ling, CD
    The exceptional oxide ionic conductivity of the high-temperature phase of bismuth oxide gives rise to a characteristic "quasielastic" broadening of its neutron scattering spectrum. We show that the oscillating form of this broadening can be fit using a modified version of a jump-diffusion model previously reserved for liquid ionic conductors. Fit parameters include a quantitative jump distance and a semiquantitative diffusion coefficient. In the case presented here, the results show that diffusion is isotropic (liquid-like) even though some directions present shorter oxygen-vacancy distances, an insight corroborated by computational dynamics simulations. More broadly, the results show for the first time that quasielastic neutron scattering can be directly analyzed to yield quantitative insights into the atomic-scale mechanisms of solid-state ionic conduction, even when the diffusing species is a coherent neutron scatterer such as oxygen. This shows its power as a tool for studying functional solid-state materials, notably for solid-oxide fuel cells and, potentially, lithium-ion batteries. © 2017 American Chemical Society.
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    Type II Bi1-xWxO1.5+1.5x: a (3 + 3) – dimensional commensurate modulation that stabilises the fast ion conducting delta phase of bismuth oxide
    (Australian Institute of Nuclear Science and Engineering, 2016-11-29) Wind, J; Auckett, JE; Withers, RL; Piltz, RO; Mole, RA; Ling, CD
    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.