THERMODYNAMICS OF THE OXIDE SOLID-SOLUTION (COXFE1-X)1-DELTA-O AT 1200-DEGREES-C

Thermogravimetric experiments have been performed to establish the stability limit of the solid solution (CoxFe1-x)1-DELTAO with x greater-than-or-equal-to 0.6 towards metallic alloys at 1200-degrees-C. The effect of the nonstoichiometry of the solid solution (CoxFe1-x)1-DELTAO on the activity of their components, a(CoO) and a(FeO), at 1200-degrees-C has been calculated. The activities were calculated using Gibbs-Duhem relations and the free energy of mixing in the solid solution due to nonstoichiometry changes was obtained. In calculations of phase equilibria in iron-containing, nonstoichiometric oxide solid solutions of the type (AxFe1-x)1-DELTAO, sometimes it has been assumed that the nonstoichiometry of such solid solutions can be described by assuming that only the disorder of the component FeO contributes to it and that this contribution is proportional to the iron content at given temperature, pressure and oxygen activity. In this paper, the errors in calculations of the free energy of mixing of solid solutions caused by using this assumption are determined. With the activities of the oxide components in the rock-salt structure phase and in the spinel phase, the miscibility gap between these two phases has been determined.