Electronic and Elastic Influences on the Energetics of Alloy Formation

Abstract

Abstract Palladium and platinum alloys have turned out to be a useful subject for relating thermodynamic properties with electronic and elastic quantities of the components. The mixing behavior of a number of interstitial and substitutional alloys exhibits analogous characteristics and may be interpreted by means of the same model. – Two effects are discussed to account for nonideal properties. An “electronic” effect is based on the observation that a number of alloy components behave as electron donors with respect to Pd and Pt. The charge transfer gives rise to a bonding contribution which is determined by the Fermi levels of the components and the valency of the solute. Extremely negative deviations from ideal mixing behavior have been predicted and observed upon adding unnoble components such as Al, Zr, Th or rare earth metals. For a number of Pd alloys the rise of the Fermi energy can be described as a function of the valence electron concentration only, irrespective of the particular donor. – An “elastic” effect is associated with the lattice distortion brought about by the dissolved particles. Models based on continuum elastic considerations are discussed in order to estimate the strain energy at infinite dilution. This quantity is correlated to the pair interaction energy in the quasichemical approach, which was used to calculate the concentration dependence of the elastic contribution. – Based on the underlying assumptions, the thermodynamic excess quantities can be separated in an electronic and an elastic part. These ideas are quite general and should be useful in a number of other metallic solutions.