Thermodynamics of intermetallics through emf measurements under effusion conditions

A general treatment is performed for a complete description of the thermodynamics of galvanic cells PtMe″,Me″zFjCaF2s.c.MehMe′k,MelMe′m,MexFyPt type working under effusion conditions where the vaporization of Me″ is considered. The treatment has been applied to three Me2Ni17Niss systems with Me≡ Eu, Tb, Ho and MehMe′k is the solid solution, Niss, produced by the eutectic reaction L↔Me2Ni17s+Niss. From the emf vs T experimental data, ΔfH298θ and ΔfST̄θ have been calculated. The ΔfH298θ values for Tb2Ni17(s) and Ho2Ni17(s) are, respectively, −16.76±0.06 and −18.8±0.1 kJ/mol atoms. In the same order, the ΔfST̄θ values are −1.27±0.06 and −1.81±0.09 JK−1/ mol atoms, respectively, at T=1023 and 1085 K. For Eu2Ni17(s), ΔfHT̄θ and ΔfST̄θ are, respectively, −48±4 kJ /mol atoms and 2.0±0.2 JK−1/mol atoms, both the values at T=910 K. The latter results should be considered unexpected and the reason for this is extensively discussed.