THERMODYNAMIC INVESTIGATION OF PT-GE AND PD-GE BINARY-ALLOYS

The enthalpy of formation, DELTAH(f), of Pt-Ge and Pd-Ge alloys was measured in the temperature range 1270-1680 K with respect to composition by direct reaction calorimetry (drop method) with the help of high temperature Calvet-type calorimeters. The results agree well with our previous measurements. The enthalpies of formation and melting of the PtGe and Pd2Ge intermediate phases were also obtained. Furthermore, we determined also some phase boundaries from the breaks of the DELTAH(f)(x) curves in agreement with the published phase diagram. From our enthalpy data and those of the free enthalpy of formation of the melts determined by Knudsen cell effusion mass spectrometry we were able to derive also the thermodynamic properties of PtGe and Pd2Ge at their melting points. These lead to the calculation of solid compound-liquid domains in agreement with the published ones and our experimental determinations. The thermodynamic behaviour of the two melts is characterized by strong negative deviations to ideality, suggesting strong chemical short-range order. The stoichiometries of the associates are strongly connected with the highest melting intermediate compounds of both binaries, i.e. Pd2Ge and PtGe. Their stability seems very high, since the excess heat capacities of the liquid phases are particularly low. Such a conclusion is in agreement with the low values of the entropy of melting of the compounds when compared with those deduced from additivity.