Thermochemical and structural stability of A- and B-site-substituted perovskites in hydrogen-containing atmosphere

The thermochemical and structural stability of complex perovskites A _1-xA'_xB1-y-zB'_yB''_zO _3±δ (A, A' ) La, Sr and B, B', B'' ) Ni, Mn, Fe, Co) was explored in H2-Ar atmosphere (5% H2-95% Ar) in a wide temperature range by thermogravimetric analysis, differential thermal analysis, XRD, and HRTEM. All perovskites showed good thermochemical stability in a temperature range of 25-300 °C. Reduction of the perovskites occurs at temperatures higher than 300 °C and can be interpreted as a multistep process. At the initial stage of exposure to H₂-Ar, a small weight gain was observed. This might indicate direct sorption of hydrogen into the lattice, forming hydride-oxide phases. On the other hand, the oxide lattice could reduce to form water, and then, the evolved water is reincorporated into the lattice to give a small weight gain. This is followed by dramatic weight loss. Water was found to be the main gaseous product formed during reduction. Complex perovskites, depending upon composition, rapidly lose up to 6-12 mol % of the lattice oxygen, which is accompanied by phase or structural transformations in the solid. Further mechanism and kinetics of reduction strongly depend on temperature. The rate of reduction at intermediate temperatures (500-700 °C) becomes slow, probably due to a local stabilization of La(OH)3 in extremely humidified hydrogen-containing atmosphere. The complete reduction of perovskites can occur at 800 °C. On long-term annealing, the perovskite containing three transition elements and Sr on the B and A sublattices, respectively, showed better thermochemical stability in hydrogen-containing atmosphere. The results suggest that the presence of structural defects and their mobility in the oxygen sublattice are important factors determining the thermochemical stability of perovskites.