STANDARD ENTHALPIES OF FORMATION OF COHF AND NIHF

The standard enthalpies of formation of the intermetallic compounds CoHf and NiHf have been determined by solute-solvent drop calorimetry at 1473 ± 2 K. The following values are reported: CoHf, -94.9 ± 6.1 kJ/mol; NiHf, -118.5 ± 4.9 kJ/mol. These results are compared with predicted values and data for the corresponding compounds of Co and Ni with Ti and Zr and Fe with Ti from high temperature direct synthesis calorimetry. Comparisons are also made with our recently published data for the equi-atomic alloys of Ti, Zr, and Hf with the 4d and 5d metals Pd, Rh, Ru, Pt, and Ir. These comparisons give further evidence for the existence of systematic analogies in trends between the 3d and 5d compounds that are not shared by the 4d compounds.