Unlocking the Potential of Palmierite Oxides: High Oxide Ion Conductivity via Induced Interstitial Defects

Hexagonal perovskite derivatives such as Ba₇Nb₄MoO₂₀ and Ba₃NbMoO_8.5 have recently been reported to exhibit high oxide ion conductivity and have potential applications in next-generation solid oxide fuel cells. In contrast, Ba₃V₂O₈ and Sr₃V₂O₈ that crystallize with the structurally related palmierite structure show oxide ion conductivities orders of magnitude lower. Here we use design principles to enhance the oxide ion conductivity in palmierites. By replacing V⁵⁺ with two cations that are known to display flexible coordination (Mo⁶⁺ and Ti⁴⁺) and manipulating the ratio of Mo⁶⁺:Ti⁴⁺ to insert interstitial oxygen, a high oxide ion conductivity of 3.96 × 10^-3 S cm^-1 at 600 °C is observed in Ba₃Ti_0.9Mo_1.1O_8.1, two orders of magnitude higher than previously reported in palmierites. The oxide ion conductivity of Ba₃Ti_0.9Mo_1.1O_8.1 is also higher than that previously reported for both Ba₇Nb₄MoO₂₀ and Ba₃NbMoO_8.5 at 600 °C. Introducing interstitial oxygen into the [BaO₂₊] layer results in a change in the oxide ion transport from a cog-wheel type motion to an interstitialcy mechanism, demonstrating that palmierites are flexible to doping strategies via the introduction of either vacancies or oxide ion interstitials.