Energetics of Rare Earth Manganese Perovskites A_1-xA′_xMnO₃ (A=La, Nd, Y and A′=Sr, La) Systems

High temperature reaction calorimetry using molten lead borate as solvent has been used to study the thermochemistry of NdMnO₃, YMnO₃, La_1-xSr_xMnO₃ (with 0<x<0.5), and Ln_0.5Ca_0.5MnO₃ (with Ln=La, Nd, Y). The enthalpies of formation of these multicomponent oxides from their binary constituents have been calculated from the measured enthalpy of drop solution. The energetic stability of the perovskite depends on the size of the A cation. The enthalpy of formation of YMnO₃ (smallest A cation) is more endothermic than those of NdMnO₃ and LaMnO₃. The energetics of the perovskite also depends on the oxidation state of the B site's ions. In the La_1-xSr_xMnO₃ system, the energetic stability of the structure increases with the Mn⁴⁺/Mn³⁺ ratio. The new values of the enthalpies of oxidations, with reliable standard entropies, were used to plot the phase stability diagram of the lanthanum-manganese-oxygen system in the temperature range 300-1100 K. The LaMnO₃/MnO phase boundary evaluated in this study agrees with the one published by Atsumi et al. calculated from thermogravimetric and conductivity measurements.