Abstract: The electromotive force(EMF) values of the cell (－)Pt|Ir|Al_0.85Sn_0.15, Na₃AlF₆|CaF₂|Al_xFe_1－x(x=0.23～0.33), Na₃AlF₆|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 Fe₃Al with DO₃-ordered nonstoichiometric intermetallic compounds were derived. The partial molar thermodynamic functions ΔG_Al、ΔH_Al、ΔS_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 (xAl=0.25).

Key words: Electromotive force (EMF), CaF₂, Fe₃Al, Activity, Thermodynamic factor