Enthalpies of Formation of La1−xAxMnO3±δ (A=Ca and Sr) Measured by High-Temperature Solution Calorimetry

Abstract

High-temperature reaction calorimetry using molten lead borate as a solvent has been used to study the thermochemistry of La1−xAxMnO3 (A=Ca and Sr, 0≤x≤1). The structural properties and the oxygen stoichiometry of the calcined powders were characterized by powder X-ray diffraction, thermogravimetry, and iodometry. The enthalpy of formation of the materials from their binary constituents has been calculated from measured enthalpy of solutions. The enthalpy of formation of La1−xCaxMnO3 from the component oxides becomes more exothermic with increasing substitution level x due to the increasing valence state of Mn, and demonstrates a nearly linear dependence of the average valence state of manganese. La1−xSrxMnO3 has a significantly more exothermic enthalpy of formation from the binary constituents than the Ca-substituted materials, and demonstrates a negative deviation from ideal behavior. The present thermodynamic data are discussed in relation to the Goldschmidt tolerance factor for the perovskite structure. Finally, the phase stability of Sr- and Ca-substituted LaMnO3 is addressed in relation to the application of these materials in solid oxide fuel cells.