Structure and transport properties of proton-conducting BaSn0.5In0.5O2.75 and A-site substituted Ba0.9Ln0.1Sn0.5In0.5O2.8 (Ln = La, Gd) oxides

In this work, characterization of BaSn0.5In0.5O2.75, Ba0.9La0.1Sn0.5In0.5O2.8 and Ba0.9Gd0.1Sn0.5In0.5O2.8 perovskite-type oxides is reported concerning crystal structure, presence of protons in hydrated samples and transport properties of dense pellets in dry and wet atmospheres at elevated temperatures. Compounds possess cubic structure at room temperature, which is in accordance with Goldschmidt's tolerance factor t being close to 1. Introduction of smaller Ln partially substituting Ba results in a decreased unit cell volume. Upon hydration in wet air unit cell of the materials enlarge, with magnitude of the effect correlated with chemical composition. High temperature X-ray measurements in synthetic air revealed release of water from hydrated materials occurring above 200 °C, and associated non-monotonous dependence of the respective unit cell parameter. No significant influence of dopants is found concerning thermal expansion of the studied oxides. Performed thermogravimetric measurements on hydrated and dried samples with supplementary mass spectroscopy analysis are in agreement with structural results, confirming presence of water in the hydrated compounds, but also show CO2 release during heating. Electrochemical impedance measurements, conducted for densified pellets in dry, H2O- and D2O-containing synthetic air, indicate an appearance of proton conductivity, with different dependence of bulk and grain boundary components on chemical composition. Supplementary electrical conductivity relaxation experiments, which allowed calculating diffusion and surface coefficients are also provided.