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 A 3 OhTd 2 O 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 ABO 3 or brownmillerite A 2 B 2 O 5 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 Ba 3 YGa 2 O 7 ( P 2/ 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 P 112 1 / 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 Ba 3 YGa 2 O 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 Ba 3 YGa 2 O 7.5 retaining proton conductivity at high temperatures. Ba 2.9 La 0.1 YGa 2 O 7.55 and Ba 3 YGa 1.9 Ti 0.1 O 7.55 appear to be dominant oxide ion conductors, with conductivities an order of magnitude higher than that of the parent compound.