THERMODYNAMIC INVESTIGATION OF THE PD-PT-GE TERNARY ALLOYS AT 1269-K

The enthalpy of formation, DELTA(f)H, of the Pd-Pt-Ge alloys was measu red at 1269 K with respect to composition by direct reaction calorimet ry (drop method) with the help of high-temperature Calvet-type calorim eters. The measurements were performed at four values (0.10, 0.25, 0.5 0 and 0.75) of R = x(Pt)/x(Pt) + x(Ge) and one (0.60) of R' = x(Pd)/X( Pd) + x(Ge). From the breaks of the enthalpy curves with respect to co mposition for given R and R' values we obtained evidence for the exist ence of a series of Pd2Ge-Pt2Ge solid solutions as well as the boundar ies of the liquid + solid solution domains. We also deduced the enthal py of formation of the Pd2Ge-Pt2Ge solution with respect to compositio n. The Pd2Ge-Pt2Ge pseudo-binary can be considered as ideal. The therm odynamic behaviour of the melts is characterized by strong negative de viations from ideality, as was also the case for the Pd-Ge and Pt-Ge b inaries. The isoenthalpic curves at 1269 K show that the minimum of th e enthalpy lies on a straight line from pure Pd2Ge to pure PtGe.