Expanded chemistry and proton conductivity in vanadium-substituted variants of γ-Ba4Nb2O9

Abstract

We have substantially expanded the chemical phase space of the hitherto unique γ- Ba4Nb2O9 type structure by designing and synthesising 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 characterised 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.