Oxide Ion and Proton Conductivity in a Family of Highly Oxygen-Deficient Perovskite Derivatives

Functional oxides showing high ionic conductivity have many important technological applications. We report oxide ion and proton conductivity in a family of perovskite-related compounds of the general formula A₃OhTd₂O_7.5, where Oh is an octahedrally coordinated metal ion and Td is a tetrahedrally coordinated metal ion. The high tetrahedral content in these ABO_2.5 compositions relative to that in the perovskite ABO₃ or brownmillerite A₂B₂O₅ structures leads to tetrahedra with only three of their four vertices connected in the polyhedral framework, imparting a potential low-energy mechanism for O^2- migration. The low- and high-temperature average and local structures of Ba₃YGa₂O₇ (P2/c, a = 7.94820(5) Å, b = 5.96986(4) Å, c = 18.4641(1) Å, and β = 91.2927(5) ° at 22 °C) were determined by Rietveld and neutron pair distribution function (PDF) analysis, and a phase transition to a high-temperature P112₁/a structure (a = 12.0602(1) Å, b = 9.8282(2) Å, c = 8.04982(6) Å, and γ = 107.844(3)° at 1000 °C) involving the migration of O^2- ions was identified. Ionic conductivities of Ba₃YGa₂O_7.5 and compositions substituted to introduce additional oxide vacancies and interstitials are reported. Most phases show proton conductivity at lower temperatures and oxide ion conductivity at high temperatures, with Ba₃YGa₂O_7.5 retaining proton conductivity at high temperatures. Ba_2.9La_0.1YGa₂O_7.55 and Ba₃YGa_1.9Ti_0.1O_7.55 appear to be dominant oxide ion conductors, with conductivities an order of magnitude higher than that of the parent compound.