Enhanced Oxide Ion Diffusion by Lanthanum Substitution in the Palmierite Sr3–3xLa2xV2O8 via Increased Tetrahedral Distortion and Cation Vacancies

Abstract

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_3–3xLa_2xV₂O₈ (x = 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 (x = 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.