Thermodynamics of Fe-Rich Intermetallics along the Rare Earth Series

Abstract

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 CaF 2 single crystal as the electrolyte. The standard enthalpy and entropy of formation of RE 2 Fe 17 intermetallics have been obtained and compared with previous results found for the RE 2 Ni 17 intermetallics. The enthalpy of formation of RE 2 Fe 17 intermetallics is decidedly less exothermic than the enthalpy of formation of RE 2 Ni 17 with the exception of the value of Pr 2 Fe 17, which is endothermic. The RE 2 Fe 17 entropy of formation is always positive contrary to the values of RE 2 Ni 17 . For comparison purposes, the thermodynamic data of Y 2 Fe 17 and Dy 2 Co 17 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)·10 -4 for Er to (9.2 ± 0.2)·10 -4 for Ho. The values of the thermodynamic activity of RE coexisting between the RE 2 Fe 17 -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 RE 2 Fe 17 intermetallics is practically independent of the dilatation term contrary to the RE 2 Ni 17 intermetallics.