Activities in the spinel solid solution, phase equilibria and thermodynamic properties of ternary phases in the system Cu-Fe-0

Abstract

A review of the structural and thermodynamic information and phase equilibria in the Cu-Fe-O system suggested that a consistent, quantitative description of the system is hampered by lack of data on activities in the spinel solid solution CuFe₂O₄-Fe₃O₄. Therefore the activity of Fe₃O₄ in this solid solution is derived from measurements of the oxygen potentials established at 1000°C by mixtures containing Fe₂O₃ and spinel solid solutions of known composition. The oxygen pressures were measured manometrically for solid solutions rich in CuFe₂O₄, while for Fe₃O₄-rich compositions the oxygen potentials were obtained by an emf technique. The activities show significant negative deviations from Raoult's law. The compositions of the spinel solid solutions in equilibrium with CuO + CuFeO₂ and Cu + CuFeO₂ were obtained from chemical analysis of the solid solution after magnetic separation. The oxygen potential of the three-phase mixture Cu + CuFeO₂ + Fe₃O₄(spinel s.s.) was determined by a solid oxide galvanic cell. From these measurements a complete phase diagram and consistent thermodynamic data on the ternary condensed phases, CuFeO₂ and CuFeO₂O₄, were obtained. An analysis of the free energy of mixing of the spinel solid solution furnished information on the distribution of cations and their valencies between the tetrahedral and octahedral sites of the spinel lattice, which is consistent with X-ray diffraction, magnetic and Seebeck coefficient measurements.