A structural study of the proton conducting B-site ordered perovskite Ba₃Ca_1.18Ta_1.82O_{8 .73}

The proton conducting material Ba3Ca1.18Ta1.82O8.73 (BCT18) was synthesized and characterized using diffraction methods and thermal analysis. It was shown that BCT18 is structurally similar to its niobium analogue (BCN18). At synthesis temperatures up to 1500 oC however, BCT18 forms a mixture of Ca- and Ta-site ordered phases, with bot 1:1 type and 1:2 type ordering. The phase ratio seems to depend solely on the synthesis conditions, with 1:1 type ordering being the dominant form in most cases. Thermal treatment in vacuum, wet and dry hydrogen, and CO2 suggests that both forms contain defects (CA’’’TA) and V..o) allowing the material to absorb water and CO2. The uptake and the release of H2O and of CO2 are all reversible, as evidenced by x-ray diffraction studies and thermal analysis, suggesting that the molecules are present as structural defects (Oho and Cox30), rather than surface species or separate hydroxide or carbonate phases. Solid state lH nuclear magnetic resonance also confirms the presence of protons and the peak broadening suggests that they are mobile at room temperature.