Abstract: The equilibrium p-c-T data were determined by a microbalance in a vacuum system at hydrogen pressures up to 3.5 MPa on four Pd-In alloys between 3.64 and 14.53 at. % In as well as on Pure Pd. The results are similar to those obtained on the analogous alloys Pd-Ag and Pd-Sn and are interpreted in the same manner. In has two opposing effects upon the solubility of H in the Pd alloys, i.e., increase of the Fermi energy upon the donation of valence electrons and a lattice dilatation. At valence electron concentrations below 0.5, the lattice dilatation predominates and the H solubility is increased with addition of In. At valence electron concentrations above 0.5, the influence of the elevated Fermi energy predominates and the H solubility is lowered by addition of In. The molar enthalpy of Pd hydriding at infinite dilution is strongly increased with increasing In content, but the molar entropy is not nearly as much affected by the Presence of In. The excess chemical potential of hydrogen at small hydrogen concentrations and at a given temperature increases with an increase of In content. The apparent H-H attractive interaction energy, W H-H, is decreased by the presence of In. The trend is also similar to that found for the Pd-Ag-H and Pd-Sn-H system.

Key words: Pd-In alloy, valence electron, lattice dilatation, Fermi energy