Thermodynamics of BaCa_{(1 + y)/3}Nb_{(2 − y)/3}O_{3 − δ}·xH₂O proton-conducting perovskites

Complex perovskites BaCa_{(1 + y)/3}Nb_{(2 − y)/3}O_{3 − δ} (y = 0, 0.18 and 0.5) with 1:1-ordered structure ($Fm\overline{3}m$ space group) were synthesized by standard ceramic technique. The enthalpy increments for these oxides were measured in the temperature range of (295–1273) K by drop calorimetry, and experimental points were fitted with an integral of a single Einstein term. The hydration enthalpies for BaCa_(1.18)/3Nb_(1.82)/3O_2.91 (BCN18), directly measured by calorimetry, were found to be equal to (− 86.7 ± 4.2) kJ (mol H₂O)⁻¹ and (− 88.8 ± 4.0) kJ (mol H₂O)⁻¹ at (673.8 ± 0.8) K and (872.8 ± 1.1) K, respectively. The calorimetrically determined enthalpies of hydration were found to be in agreement with those calculated using water vapor pressure–temperature–composition data. The increase in the off-stoichiometry index y in BaCa_{(1 + y)/3}Nb_{(2 − y)/3}O_{3 − δ} leads to more exothermic hydration heats. The enthalpies of formation from elements, determined by drop solution calorimetry for dehydrated BaCa_{(1 + y)/3}Nb_{(2 − y)/3}O_{3 − δ} with y = 0, 0.18 and 0.5, become less exothermic with increasing calcium content, and are equal to (− 1577.5 ± 14.5) kJ mol⁻¹, (− 1533.6 ± 14.8) kJ mol⁻¹ and (− 1492.2 ± 14.6) kJ mol⁻¹, respectively. The thermodynamic data obtained allow calculating the enthalpies of formation and enthalpy increments for BaCa_{(1 + y)/3}Nb_{(2 − y)/3}O_{3 − δ} with arbitrary water content.