The present stage of thermodynamic development is characterized by the design of more specific thermodynamic methods and their application to development and processing of materials, together with a systematic accumulation and utilization of the data of thermodynamic chemistry. Through its wide of applicability and the range of information that it can provide, the solid electrolyte electrochemical technique is demonstrably a valuable tool in investigating both phase diagrams and thermodynamic properties of the multicomponent systems.
It is recognized that the physical properties of the perovskite-type compounds in the La-Mn-O system are largely dependent on the defect chemistry. Although the knowledge of the nonstoichiometric behavior has increased during the years, the relationships between the nonstoichiometry and thermochemical stability of many compounds has not been elucidated yet.
The aims of this work were as follows:
The investigation of the correlation between the nonstoichiometry and the thermodynamic properties for the LaMnO3±δ perovskite-type compounds at temperatures between 1073 and 1373 K and in a reducing atmosphere of 10-5 Pa. The thermodynamic properties represented by the relative partial molar free energies, enthalpies and entropies of oxygen dissolution in the perovskite phase, as well as the partial pressures of oxygen were determined as a function of temperature and composition. The influence of the oxygen stoichiometry change on the thermodynamic properties was examined using the data obtained by a coulometric titration technique coupled with EMF measurements. The obtained results are discussed being related with the variation of the predominant defects in the perovskite structure.
The examination of the solid state equilibrium relationships in order to evidence the possibility of formation of the La2MnO4 phase and of the La2MnO4.15 compound having a K2NiF4 type structure. The plausible reactions for the formation of these compounds are discussed by comparing the experimental data with the theoretical results obtained by Yokokawa by using an empirical correlation between the stabilization energy and the Goldschmidt tolerence factor.