Ba₇Nb₄MoO₂₀: A Proton or Oxide Ion Conductor?

Ba₇Nb₄MoO₂₀ is a recently emerging fast oxide ion conductor, attracting increasing attention. It was reported to also show proton conduction in a humid atmosphere, which is, however, opposite to the results in this work. Theoretical calculations indicate that protons can hardly move, being stabilized in the perovskite blocks or trapped in the palmierite layers, which is in line with the experimental evidence of almost no H₂O/D₂O isotope effect and a negligibly small transport number of protons by EMF measurements up to 800 °C. It is thereby clear that Ba₇Nb₄MoO₂₀ is almost a pure oxide ion conductor even though it can be hydrated upon exposure to moisture. The hydration in fact stabilizes interstitial oxide ions (O5 site) in the palmierite layers by forming Nb₂O₉H units, leading to reduced bulk conductivity in the pristine and slightly Nb-rich compositions at <400 °C. However, by heating up over 400 °C, the bulk conductivity in the wet atmosphere surpasses that in the dry one, benefitting from an injection of protons into the perovskite blocks to release free O5 ions for oxide ion conduction, indicating that hydration may provide extra oxide ions to enhance the oxide ion conduction.