Thermodynamic Properties of Pd‐X‐Alloys, with X = Gd, Y, Ce

Abstract

AbstractElectromotive force measurements on cells with a CaF2 solid electrolyte were used to determine the activities of Gd, Y, and Ce in Pd‐rich alloys between 700°C and 800°C. The thermodynamic properties of these alloys are characterized by extreme negative deviations from ideal mixing behavior, up to −400 kJ/mol for the partial molar excess Gibbs energy of Ce and up to −320 kJ/mol for Gd and Y. The high thermodynamic stability of these alloys is attributed to a transfer of valence electrons to the electron gas of the alloy. At infinite dilution the resulting bonding contribution to the partial excess free energy is determined by the difference of the Fermi levels of the components and the valency of the solute. Including the size difference of the components as an additional effect, the mixing behavior of the alloys can be well described. – Within the homogeneity ranges of the intermetallic phases Pd3Y and Pd3Gd the activities of the unnoble components increase by more than ten orders or magnitude.