Expanded chemistry and proton conductivity in vanadium-substituted variants of γ-Ba4Nb2O9
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Date
2021-09-09
Journal Title
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Publisher
American Chemical Society
Abstract
We have substantially expanded the chemical phase space of the hitherto unique γ-Ba4Nb2O9 type structure by designing and synthesizing stoichiometric ordered analogues γ-Ba4V1/3Ta5/3O9 and γ-Ba4V1/3Nb5/3O9 and exploring the solid-solution series γ-Ba4VxTa2–xO9 and γ-Ba4VxNb2–xO9. Undoped Ba4Ta2O9 forms a 6H-perovskite type phase, but with sufficient V doping the γ-type phase is thermodynamically preferred and possibly more stable than γ-Ba4Nb2O9, forming at a 200 °C lower synthesis temperature. This is explained by the fact that Nb5+ ions in γ-Ba4Nb2O9 simultaneously occupy 4-, 5-, and 6-coordinate sites in the oxide sublattice, which is less stable than allowing smaller V5+ to occupy the former two and larger Ta5+ to occupy the latter. The x = 1/3 phase γ-Ba4V1/3Ta5/3O9 shows greatly improved ionic conduction compared to the x = 0 phase 6H-Ba4Ta2O9. We characterized the structures of the new phases using a combination of X-ray and neutron powder diffraction. All compositions hydrate rapidly and extensively (up to 1/3 H2O per formula unit) in ambient conditions, like the parent γ-Ba4Nb2O9 phase. At lower temperatures, the ionic conduction is predominately protonic, while at higher temperatures it is likely other charge carriers make increasing contributions.© 2021 American Chemical Society
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Keywords
X-ray diffraction, Transition elements, Electric conductivity, Thermal gravimetric analysis, Absorption, Neutron diffraction, Perovskite
Citation
Brown, A. J., Schwaighofer, B., Avdeev, M., Johannessen, B., Evans, I. R., & Ling, C. D. (2021). Expanded chemistry and proton conductivity in vanadium-substituted variants of γ-Ba4Nb2O9. Chemistry of Materials, 33(18), 7475-7483. doi:10.1021/acs.chemmater.1c02340