Enhanced Oxide Ion Diffusion by Lanthanum Substitution in the Palmierite Sr_3–3xLa_2xV₂O₈ via Increased Tetrahedral Distortion and Cation Vacancies

High Resolution Image Download MS PowerPoint Slide Promising ionic conductivity has previously been reported in the palmierite oxide Sr 3 V 2 O 8 . Oxide-ion diffusion within this system occurs via the “cog-wheel” mechanism where rotation of VO 4 and oxygen deficient VO 3 units result in the formation of V 2 O 7 dimers which continuously break and reform. During this process an oxide ion from a VO 4 tetrahedra can move to the vacant site of the VO 3 group, facilitating the movement of oxide-ions throughout the structure. Here, we report the electrical and structural properties of the series Sr 3–3 x La 2 x V 2 O 8 ( x = 0.00–0.25). Combined neutron diffraction and atomistic simulations reveal that substituting Sr 2+ with La 3+ results in ordered cation vacancies, an elongation of the apical V–O1 bond and an increase in the distortion of the VO 4 tetrahedra, which together enhances the cog-wheel-like rotational dynamics of the VO 4 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 2.55 La 0.3 V 2 O 8 ( 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 Sr 3 V 2 O 8 . The results demonstrate that palmierites are highly flexible to doping strategies for improving the oxide ion conductivity.