Thermodynamic properties of nonstoichiometric intermetallic compounds FeAl with DO3 structure

The electromotive force (EMF) values of the cell (-) Pt vertical bar Ir vertical bar Al0.85Sn0.15, Na3AlF6 vertical bar CaF2 vertical bar AlxFe1-x ( x = 0.23 similar to 0.33), Na3AlF6 vertical bar Ir vertical bar 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, Al0.85Sn0.15 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 Delta G(Al), Delta H-Al, Delta S-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).