Thermodynamics of Phase Equilibria in the CoO–BaO–Fe2O3 System

The work presents a thermodynamic analysis of phase equilibria in the subsolidus region of the three-component oxide system CoO–BaO–Fe2O3. The relevance of the study is due to the growing interest in ceramic ferrites with specified magnetic and electromagnetic properties, which are used in the creation of functional composite materials. The aim of the work was to establish thermodynamically stable binary and ternary phase combinations in the CoO–BaO–Fe2O3 system based on the analysis of solid-phase exchange reactions without taking into account ternary oxide compounds. This analysis represents a simplified thermodynamic model that considers only binary oxide compounds and excludes ternary ferrite phases. Thermodynamic calculations of Gibbs energy changes for model reactions of the type “2 = 2” were performed in the temperature range 1000–1800 K using the temperature dependencies of the enthalpies and entropies of compounds. To resolve contradictions arising from the analysis of the stability of individual conjugates, the method of conjugating exchange reactions with a transition to “3 = 2” type interaction mechanisms was applied. As a result of triangulation, nine thermodynamically stable binary combinations of compounds and ten stable triple phase combinations corresponding to elementary triangles of the subsolidus structure of the system were identified. The predisposition of the CoFe2O4–BaFe12O19 compound to destabilization is demonstrated, and its structural and phase stabilization due to the formation of an equilibrium three-phase combination of CoFe2O4–CoO–BaFe12O19 is substantiated. A general rule has been formulated for analyzing the thermodynamic stability of phase combinations in exchange reactions of the type “3 = 2”. The results obtained provide a physicochemical basis for predicting the phase composition of ferrite materials and composites in any concentration range of the CoO–BaO–Fe2O3 system and can be used in the development of technologies for the reaction synthesis of new ceramic ferrites.