Thermodynamics of the Ni-Yb system

The experimental description of the Ni-rare earth systems is incomplete, with many gaps and uncertainties remaining in both the phase diagram and the thermochemical information. No thermodynamic information is available for the Ni-Yb system, except for an estimation of the enthalpy of formation of the Ni5Yb compound. In this work, we investigated the thermochemistry of the Ni-Yb intermediate phases by means of tensimetric measurements and, on a few compositions, calorimetric measurements. The equilibrium vapor pressures over two solid-phase regions were measured by Knudsen effusion-mass spectrometry and Knudsen effusion-weight loss, and the data were analyzed by the second- and the third-law methods in order to derive the enthalpy changes for the HT decomposition reactions. Furthermore, the enthalpy of formation of the NiYb compound was preliminarily determined by direct reaction calorimetry (DRC). The heats of formation of the five intermediate phases was finally obtained by a combined analysis of all the collected data. The values are as follows: Ni17Yb2, -13.9 +/- 0.3 kJ/mol atoms; Ni5Yb, -20.9 +/- 4.4 kJ/mol atoms; Ni3Yb, -26.0 +/- 4.8 kJ/mol atoms; Ni2Yb, -32.0 +/- 4.6 kJ/mol atoms; and NiYb, -28.0 +/- 2 kJ/mol atoms. The results are compared with those estimated by the Miedema's model.