Enhancement of oxide ion and proton conductivity in Sr3-xCaxV2O8 palmierites through tuning of the crystal structure

Sizeable oxide ion and proton conductivity have recently been reported in A₃V₂O₈ (A = Ba, Sr) palmierites. The solid solution Sr_3-xCa_xV₂O₈ (x = 0.0–0.2) has been synthesised and investigated by electrochemical characterisation and atomistic modelling, revealing key insights into the design principles for enhancing oxide and proton conduction in palmierites. Neutron diffraction data shows that at 20 °C there is a reduction in crystal symmetry from $R\overline{3}m$ to $C2/c$ for x ≥ 0.05. Upon heating to 600 °C, $R\overline{3}m$ symmetry is restored for all phases. An increase in both the oxide ion and proton conductivity are observed with Ca²⁺ doping. The oxide ion conductivity scales with the magnitude of displacement of the V⁵⁺ cation and VO2 bond length.