EFFECTS OF THE OXYGEN NONSTOICHIOMETRY AND Mn VALENCE CONVERSION ON THE THERMODYNAMIC PROPERTIES OF LaMnO_(3±δ) PEROVSKITE-TYPE COMPOUNDS

摘要

The thermodynamic properties of the nonstoichiometric lanthanum manganites were studied in order to reconsider the limiting composition of the LaMnO_(3±δ) phases within the temperature range of 1073-1273 K and in reducing atmosphere (10~(-5) Pa). A solid state electrochemical method was used to obtain the relative partial molar free energies, enthalpies and entropies of oxygen dissolution in the perovskite phase, as well as the equilibrium partial pressures of oxygen. The influence of the oxygen stoichiometry change on the thermodynamic properties was investigated using the data obtained by a solid state coulometric titration technique coupled with electromotive force (EMF) measurements. The results were correlated with the average Mn valence values as determined by redox titration. An attempt to verify some previously proposed defect-structure models is also made.