Expanded Chemistry and Proton Conductivity in Vanadium-Substituted Variants of γ‑Ba₄Nb₂O₉

We have substantially expanded the chemical phase space of the hitherto unique γ-Ba₄Nb₂O₉ type structure by designing and synthesizing stoichiometric ordered analogues γ-Ba₄V_1/3Ta_5/3O₉ and γ-Ba₄V_1/3Nb_5/3O₉ and exploring the solid-solution series γ-Ba₄V_xTa_2–xO₉ and γ-Ba₄V_xNb_2–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-Ba₄V_1/3Ta_5/3O₉ 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.