Abstract
Thermodynamic activities of aluminum in the iron–aluminum system were determined between about 720 and 850 K for the composition range between 23 and 33 at.% Al which includes the D0 3-ordered nonstoichiometric intermetallic compound Fe 3Al. For this purpose an electromotive force method with a single-crystal CaF 2 electrolyte was used, and the following cell arrangement was employed: (−)Pt|Ir|Al 0.85Sn 0.15, Na 3AlF 6|CaF 2|Al 1± x Fe 3± x , Na 3AlF 6|Ir|Pt (+). Activities of iron were obtained by means of a Gibbs-Duhem integration for the same composition range. The results of the activity measurements were interpreted in terms of a statistical–thermodynamic model for nonstoichiometric phases with the D0 3-superstructure based on defect formation energies from the literature. It was found that non-stoichiometry in Fe 3Al is caused by anti-structure atoms on two of the three possible sublattices.
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