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

We have substantially expanded the chemical phase space of hitherto unique γ-Ba4Nb2O9 type structure by designing and synthesizing stoichiometric ordered analogues γ-Ba4V1/3Ta5/3O9 γ-Ba4V1/3Nb5/3O9 exploring solid-solution series γ-Ba4VxTa2–xO9 γ-Ba4VxNb2–xO9. Undoped Ba4Ta2O9 forms a 6H-perovskite phase, but with sufficient V doping γ-type is thermodynamically preferred possibly more stable than γ-Ba4Nb2O9, forming at 200 °C lower synthesis temperature. This explained fact that Nb5+ ions in simultaneously occupy 4-, 5-, 6-coordinate sites oxide sublattice, which less allowing smaller V5+ to former two larger Ta5+ latter. The x = 1/3 shows greatly improved ionic conduction compared 0 6H-Ba4Ta2O9. characterized structures new phases using combination X-ray neutron powder diffraction. All compositions hydrate rapidly extensively (up H2O per formula unit) ambient conditions, like parent phase. At temperatures, predominately protonic, while higher temperatures it likely other charge carriers make increasing contributions.