Thermodynamic properties of nonstoichiometric intermetallic compounds Fe₃Al with DO₃ structure

The electromotive force(EMF) values of the cell (-) Pt | Ir | Al _0.85 Sn_0.15, Na₃AlF₆1 | CaF ₂ | Al_xFe_1-x, (x=0.23-0.33), Na ₃AlF₆ | Ir | Pt (+) were measured between 720 K and 847 K using solid galvanic cells with a single-crystal CaF₂ electrolyte. For attaining quick thermodynamic equilibrium of the cell, Al _0.85Sn_0.15 alloy was used as the reference electrode and DO₃-Fe₃Al was powdered. It was confirmed that there was no chemical transformation in the working electrode by X-ray diffraction experiments before and after EMF measurement. The Al activities in Fe ₃Al with DO₃-ordered nonstoichiometric intermetallic compounds were derived. The partial molar thermodynamic functions ΔḠA_Al, ΔH̄_Al, ΔS̄ _Al of Al were calculated. The activities and partial molar Gibbs energies of Fe in the Fe-Al alloys were derived by Gibbs-Duhem equation. The thermodynamic factor in diffusion of aluminum in the Fe₃Al phase at 750 K was calculated, and showed a maximum at a composition near the stoichiometric proportion (x_Al=0.25).