Enhanced Oxide Ion Diffusion by Lanthanum Substitution in the Palmierite Sr_{3–3<i>x</i>}La_2<i>x</i>V₂O₈ via Increased Tetrahedral Distortion and Cation Vacancies

Promising ionic conductivity has previously been reported in the palmierite oxide Sr₃V₂O₈. Oxide-ion diffusion within this system occurs via the "cog-wheel" mechanism where rotation of VO₄ and oxygen deficient VO₃ units result in the formation of V₂O₇ dimers which continuously break and reform. During this process an oxide ion from a VO₄ tetrahedra can move to the vacant site of the VO₃ group, facilitating the movement of oxide-ions throughout the structure. Here, we report the electrical and structural properties of the series Sr_3-3<i>x</i> La_2<i>x</i> V₂O₈ (<i>x</i> = 0.00-0.25). Combined neutron diffraction and atomistic simulations reveal that substituting Sr²⁺ with La³⁺ results in ordered cation vacancies, an elongation of the apical V-O1 bond and an increase in the distortion of the VO₄ tetrahedra, which together enhances the cog-wheel-like rotational dynamics of the VO₄ tetrahedra that mediate oxide ion transport. Molecular dynamics simulations further indicate that La-substitution facilitates the formation of a continuous diffusion network, leading to improved oxide ion conductivity so that Sr_2.55La_0.3V₂O₈ (<i>x</i> = 0.15) exhibits the highest bulk conductivity of 7.64 × 10^-4 S cm ^-1 at 700 °C, an order of magnitude higher than Sr₃V₂O₈. The results demonstrate that palmierites are highly flexible to doping strategies for improving the oxide ion conductivity.