Oxide Ion Conductivity, Proton Conductivity, and Phase Transitions in Perovskite-Derived Ba 3- x Sr x YGa 2 O 7.5 0 ≤ x ≤ 3 Materials.

We report the synthesis, structural characterization, and oxide ion and proton conductivities of the perovskite-related Ba Sr YGaO family. Single-phase samples are prepared for 0 ≤ ≤ 3 and show a complex structural evolution from 2/ to 2 space groups with an increase in . For 1.0 ≲ ≲ 2.4, average structures determined by X-ray and neutron powder diffraction show metrically orthorhombic unit cells, but HAADF-STEM imaging reveals this is caused by microstructural effects due to intergrowths of the Ba- and Sr-rich structure types. Variable-temperature powder diffraction studies suggest that 0 ≲ ≲ 2.4 compositions undergo a phase transition upon being heated to space group that involves disordering of the oxygen substructure. Thermal expansion coefficients are reported for the series. Complex impedance studies show that the Ba-rich samples are mixed proton and oxide ion conductors under moist atmospheres but are predominantly oxide ion conductors at high temperatures or under dry atmospheres. Sr-rich samples show significantly less water uptake and appear to be predominantly oxide ion conductors under the conditions studied.