Dual ionic conductivity in Ba3InGa2O7.5: correlating structure and electrochemical properties

Abstract

Perovskite-related materials derived from the brownmillerite-type Ba₂In₂O₅ and those with the formula A₃OhTd₂O_7.5 (A = 2⁺ cation, Oh = octahedral 3⁺ cation, Td = tetrahedral 3⁺ cation) are two distinct classes of ionic conductors. We report the synthesis of Ba₃InGa₂O_7.5, a material spanning these families that demonstrates promising electrochemical behaviour. X-ray powder diffraction analysis shows it adopts a monoclinic P2/c structure at room temperature (a = 7.9557(13) Å, b = 5.8762(9) Å, c = 18.2237(3) Å and β = 91.570(1)°) and undergoes a phase transition to space group Cmcm at 600 °C. Humidified variable temperature X-ray powder diffraction and thermogravimetric analysis demonstrate that Ba₃InGa₂O_7.5 can uptake significant amounts of H₂O without decomposition when exposed to moisture-rich atmospheres. Complex impedance studies reveal dual ionic conductivity, with either proton or oxide ion transport dominating depending on temperature and experimental conditions. Ba₃InGa₂O_7.5 exhibits the highest total conductivity for an unsubstituted A₃OhTd₂O_7.5-type material.