Proton and Oxide Ion Conductivity in Palmierite Oxides

Solid proton and oxide ion conductors have key applications in several hydrogen-based and energy-related technologies. Here, we report on the discovery of significant proton and oxide ion conductivity in palmierite oxides A₃V₂O₈ (A = Sr, Ba), which crystallize with a framework of isolated tetrahedral VO₄ units. We show that these systems present prevalent ionic conduction, with a large protonic component under humidified air (t _H ∼ 0.6-0.8) and high protonic mobility. In particular, the proton conductivity of Sr₃V₂O₈ is 1.0 × 10^-4 S cm^-1 at 600 °C, competitive with the best proton conductors constituted by isolated tetrahedral units. Simulations show that the three-dimensional ionic transport is vacancy-driven and facilitated by rotational motion of the VO₄ units, which can stabilize oxygen defects via formation of V₂O₇ dimers. Our findings demonstrate that palmierite oxides are a new promising class of ionic conductors where stabilization of parallel vacancy and interstitial defects can enable high ionic conductivity.