Energetics of formation and stability in high pressure steam of barium lanthanide cobaltite double perovskites

This study concerns energetics of formation and the stability in high water partial pressure of BaLnCo₂O_6−δ, (Ln = La, Pr, Nd, and Gd) (BLnC) and BaGd_1−xLa_xCo₂O_6−δ, where x = 0.2, 0.5, and 0.7 (BGLC) double perovskite cobaltites. Those materials are extensively studied due to their potential applications as a positrode in electrochemical devices. Therefore, their stability under such conditions is a key issue. All investigated materials are thermodynamically stable relative to binary oxides and exhibit strongly exothermic enthalpies of formation. Moreover, BaGd_0.3La_0.7Co₂O_6−δ and BaGd_0.8La_0.2Co₂O_6−δ remain the main perovskite structure up to 3 bars of water vapor at 400 °C. At higher steam pressure, reaching 10 bar at 300 °C, the partial decomposition to constituent oxides and hydroxides was observed. The BGLC compounds exhibit higher negative formation enthalpies in comparison to single-Ln compositions, which does not translate into higher chemical stability under high steam pressures since the BLnC series retained the main perovskite structure at higher temperatures as well as in higher water vapor pressures.