Refinement of thermodynamic data for LaMnO3

Abstract

The standard Gibbs energy of formation of LaMnO3 has been measured in the temperature range from 900 to 1400 K using a solid-state cell incorporating a buffer electrode. The oxygen potential corresponding to the three-phase mixture MnO + La2O3 + LaMnO3 has been measured using pure oxygen gas at 0.1 MPa as the reference electrode, and yttria-stabilized zirconia as the solid electrolyte. The buffer electrode prevented polarization of the three-phase electrode and ensured accurate data. The measured oxygen chemical potential corresponding to the decomposition of LaMnO3−δ to MnO, La2O3 and O2 gas is in fair agreement (±6 kJ mol−1) with most data available in the literature and can be represented by the equation,ΔμO2/J mol−1 (±520) = −583160 + 170.55 (T/K)Combining this with available information on oxygen potential variation with the non-stoichiometric parameter δ, the standard Gibbs free energy of formation of LaMnO3 from MnO, La2O3 and O2 was evaluated. The correction for non-stoichiometry was ∼1.6(±0.7) kJ mol−1. The heat capacity of LaMnO3 has been measured by a differential scanning calorimeter from 400 to 1060 K. Information on low-temperature heat capacity available in the literature was joined smoothly with the high-temperature data to evaluate the standard entropy of LaMnO3 at 298.15 K as 116.68 J mol−1 K−1. The “third-law” analysis of the Gibbs energy values obtained in this study yields a value of −155.93 (±0.8) kJ mol−1 for the standard enthalpy of formation of LaMnO3 from MnO, La2O3 and O2 at 298.15 K. This value provides a critical test of calorimetric data on enthalpy of formation of LaMnO3. A consistent set of thermodynamic data for LaMnO3 is presented from 298.15 to 1400 K based on the results obtained in this study and other supporting information in the literature.