Thermochemistry of perovskite-related oxides with high oxidation states: Superconductors, sensors, fuel cell materials

Perovskite-related oxides containing transition metals (Mn, Fe, Co, Ni, Cu) in high formal oxidation states are important materials as high temperature superconductors, electrochemical sensors and solid oxide fuel cell electrodes. High oxidation states are stabilized by large alkaline earth ions, Ba and Sr. Recent study by high temperature reaction calorimetry has revealed systematics in both enthalpies of formation and of oxygen incorporation in these materials. The enthalpy of formation from the oxides of a series of ternary oxides containing formal Cu3+ becomes increasingly exothermic with decreasing ionic potential, charge/size, of the counterbalancing cation, that is, in the series LaCuO3, LaSrCuO4, LaBaCuO4, NaCuO2, KCuC>2. The enthalpy of oxygen incorporation becomes more exothermic in the sequence M = Ca, Sr, Ba for the series MxLa2-xCuO4-y. The oxidation enthalpy is similar for Ba-doped lanthanum cuprates and for YBCO. This enthalpy is similar to that of the formation of the corresponding alkaline earth peroxide for both Ba and Sr, suggesting an energetic and possibly also structural relation between hole states in these multicomponent oxides and in peroxides. The doped lanthanum nickelates and cobaltates show more exothermic oxidation energetics than the cuprates. The overall energetics are governed by the competition of oxygen vacancy formation, oxidation to formal M3+, and coupled ionic substitution. Energetic parameters for these defect formation reactions are derived from calorimetric data.