Hydration Processes of the Proton-Conducting Hexagonal Perovskites Ba₇In_6–<i>x</i>Y_<i>x</i>Al₂O₁₉

The crystal structure, TG-MS, IR, and hydration thermodynamic parameters of the inherently oxygen-deficient Ba7In6–xYxAl2O19 system have been investigated. X-ray diffraction analysis showed that the Ba7In6–xYxAl2O19 (0 ≤ x ≤ 0.25) are hexagonal perovskites belonging to the P63/mmc space group. The hydrated materials exhibited the same structure with larger cell parameters. IR spectroscopy of the hydrated specimens confirmed the existence of three types of hydroxyl groups involved in different hydrogen bonds. The dissolution of water was measured by using thermogravimetric (TG) analysis. The TG measurements showed the water uptake growth with increase of x. Dehydration upon heating is a three-stage process, namely, two close effects in the temperature range 200–600 °C and a small high-temperature effect near 800 °C. A defect chemical model was developed to derive hydration thermodynamic parameters based on TG data. A thermodynamic approach to the hydration process showed that the increase in the Y3+ content was accompanied by a decrease in the enthalpy of hydration due to the enlargement in the basicity of the phases. The strategy of introducing an isovalent dopant with a larger size and lower electronegativity turned out to be favorable in terms of increasing the proton concentration and decreasing the enthalpy of hydration.