Oxide ion conductivity, proton conductivity and phase transitions in perovskite-derived Ba3–xSrxYGa2O7.5 0 x 3 materials

We report the synthesis, structural characterization, oxide ion and proton conductivities of the perovskite-related Ba3–xSrxYGa2O7.5 family. Single-phase samples are prepared for 0 x 3 and show a complex structural evolution from P2/c to C2 space groups with increasing x. For 1.0 ≲ x ≲ 2.4 average structures determined by X-ray and neutron powder diffrac-tion 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 compositions with 0 ≲ x ≲ 2.4 undergo a phase transition on heating to space group Cmcm 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 temperature or under dry atmospheres. Sr-rich samples show significantly less water uptake and appear to be predominantly oxide ion conductors under the conditions studied