STRUCTURAL AND THERMODYNAMIC PROPERTIES OF THE DEUTERIUM-PALLADIUM SOLID-SOLUTIONS SYSTEMS - D-2-[PD(PT),PD(RH), PD(PT, RH)]

Different palladium binary or ternary solid solutions with platinum an d rhodium-deuterium systems are investigated. Lattice constants of the f.c.c. solid solution Pd-100-(x+y)PtxRhy vary linearly as a function of x or y in the range 0 less than or equal to x, y less than or equal to at. 10%. Substitution of Pt for Pd induces an increase in the latt ice constant whereas substitution by Ph leads to a decrease in the lat tice parameter. The deuterium pressure-composition isotherms have been determined at several temperatures between 10 and 100 degrees C and t he enthalpy and entropy of the absorption and desorption reactions hav e been calculated. The substitutions involve. in all cases, an increas e in the plateau pressures and a decrease in the enthalpies of the rea ctions, in absolute value. The Pt substitution is anomalous in regard of the geometric model, where the larger cell volume corresponds to th e more stable deuteride (hydride). For the ternary alloys, it was noti ced that absorption pressures depend on the global substitution rate P t + Rh but not at all on the Pt/Rh ratio. Desorption pressures increas e slightly with the Pt/Ph ratio. The enthalpy of desorption is nearly constant for 5% substituted alloys, whereas it decreases for 10% subst ituted alloys until it is equal to the absorption value.