Thermodynamics of Solution of Hydrogen in Pd Cu and Pd Cu Au Solid Solution Alloys

Thermodynamic quantities for absorption of hydrogen by Pd  Cu alloys with up to atom fraction of XCu = 0.2, and by Pd  Cu  Au ternary alloys with up to (XCu + XAu) = 0.2, where XCu = XAu, have been determined at temperatures between 273 K and 423 K and hydrogen pressures up to 1000 Torr by measurements of the pressure‐composition isotherms. The hydrogen absorption characteristics of these alloys have been compared with that of the previously reported Pd – Au binary alloys. The room temperature lattice parameters of hydrogen‐free Pd – Cu – Au alloys increases with the solute content, albeit by a small amount. The relative partial molar enthalpy, ΔHH0, at infinite dilution in Pd – Cu – Au alloys increases in exothermicity with increasing (Cu + Au) content, and the alloy composition dependence of the ΔHH0 value falls near the mid‐points between those of each Pd – Cu and Pd – Au binary alloy. The relative partial molar entropy, ΔSH0, at infinite dilution in Pd – Cu – Au alloys decreases with the solute content, and the ΔSH0 values at the same alloying content are larger than that of Pd – Au alloys, but are smaller than that of Pd – Cu alloys. The [H – Cu + H – Au] pair interaction energies between the dissolved hydrogen and the substitutional solute atoms in the ternary alloys have been evaluated according to a quasi‐chemical model with zeroth order approximation by taking into account the “constant pressure” to “constant volume” correction. The hydrogen solubility behaviour at low hydrogen concentration in Pd – Cu – Au ternary alloys can be described as due to the counteraction of the repulsive H – Cu pair interaction and the attractive H – Au pair one in the Pd host lattice. The stability of the β‐phase hydride at a given temperature in Pd – Cu – Au alloys decreases with the solute content, and the alloy composition dependence of the stability is also placed near the mid‐points between those for each Pd – Cu and Pd – Au binary alloy.