Abstract
Different palladium binary or ternary solid solutions with platinum and rhodium-deuterium systems are investigated. Lattice constants of the f.c.c. solid solution Pd100−(x+y)PtxRhy vary linearly as a function of x or y in the range 0 ⩽ x, y ⩽at. 10%. Substitution of Pt for Pd induces an increase in the lattice constant whereas substitution by Rh leads to a decrease in the lattice parameter. The deuterium pressure-composition isotherms have been determined at several temperatures between 10 and 100°C and the enthalpy and entropy of the absorption and desorption reactions have been calculated. The substitutions involve, in all cases, an increase in the plateau pressures and a decrease in the enthalpies of the reactions, in absolute value. The Pt substitution is anomalous in regard of the geometric model, where the larger cell volume corresponds to the more stable deuteride (hydride). For the ternary alloys, it was noticed that absorption pressures depend on the global substitution rate Pt + Rh but not at all on the Pt/Rh ratio. Desorption pressures increase slightly with the Pt/Rh ratio. The enthalpy of desorption is nearly constant for 5% substituted alloys, whereas it decreases for 10% substituted alloys until it is equal to the absorption value.
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