Nonstoichiometry and thermodynamics of the solid solution (Fe, Mn)1 − ΔO at 1200°C

Abstract A detailed analysis of the effect of the nonstoichiometry of the solid solution (Fe, Mn) 1 − Δ O on the activity of their components, a FeO and a MnO , at 1200°C has been carried out. With nonstoichiometry data available from new thermogravimetric measurements of Δ in (Fe, Mn) 1 − Δ O changes in the free energy of mixing in the solid solution due to nonstoichiometry changes were calculated via the integration of Gibbs-Duhem equations. In calculations of phase equilibria in iron-containing, nonstoichiometric oxide solid solutions of the type (Fe 1 − x A x ) 1 − Δ O, 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 this assumption are determined. Furthermore, the log a O 2 vs composition phase diagram of Fe-Mn-O at 1200°C is calculated.