SOLUBILITY AND THERMODYNAMICS OF HYDROGEN IN HOMOGENEOUS FCC PD-PT ALLOYS

Hydrogen solubility data have been determined for homogeneous f.c.c. Pd-Pt alloys. In comparison with pure Pd, alloying with Pt leads to a decrease in the dilute phase hydrogen solubility at a given p(H2) and an increase of the plateau pressures. This makes this alloy-H system an exception to the classification of Pd alloys with regard to their hydrogen absorption behavior as contracted or expanded; the former type of alloys have lower dilute phase solubilities and greater plateau pressures than Pd, and the latter have larger dilute phase solubilities and lower plateau pressures. The enthalpies for hydrogen solution at infinite dilution of hydrogen do not change much with X(pt) but the corresponding entropies do change in such a way which would indicate that only interstices with six nearest neighbor Pd atoms are occupied. The enthalpies for hydride formation decrease in magnitude with X(pt).