Thermodynamics of Fe-Rich intermetallics along the rare earth series

A set of thermodynamic properties of the Fe-rich part of the Fe/rare earth (RE) systems is presented for almost all the elements in the rare earth series. This set of data comes entirely from electromotive force (emf) vs T experimental measurements obtained by galvanic cells with a CaF2 single crystal as the electrolyte. The standard enthalpy and entropy of formation of RE2Fe17 intermetallics have been obtained and compared with previous results found for the RE2Ni17 intermetallics. The enthalpy of formation of RE2Fe17 intermetallics is decidedly less exothermic than the enthalpy of formation of RE2Ni17 with the exception of the value of Pr2Fe17, which is endothermic. The RE2Fe17 entropy of formation is always positive contrary to the values of RE2Ni17. For comparison purposes, the thermodynamic data of Y2Fe17 and Dy2CO17 have also been determined. The RE solubility in Fe has been evaluated by the shift of the bcc Fe(110) plane spacing with respect to pure Fe as shown by corresponding X-ray diffraction (XRD) data. Along the RE series, the atomic fraction of RE in the Fe solid solution changes from (1.2 +/- 0.1)center dot 10(-4) for Er to (9.2 +/- 0.2)center dot 10(-4) for Ho. The values of the thermodynamic activity of RE coexisting between the RE2Fe17-rich phase and the Fe solid solution are reported along the series, and the related partial excess free energy is given as electronic and dilatation contributions. The electronic partial excess free energy of RE2Fe17 intermetallics is practically independent of the dilatation term contrary to the RE2Ni17 intermetallics.