Fermi surface incommensurate nestings and phase equilibria in Cu-Pd alloys

Abstract

The Cu-Pd phase diagram shows two low-temperature ordered phases corresponding to B2 and L1 2 superlattices or, regardless of the site occupation, to bcc and fcc geometrical arrangements. We show how the phase equilibria between these intermetallic compounds can be understood by introducing a hypothetical bcc random-alloy phase and a three-step transformation from B2 to random bcc to random fcc to L1 2 . The two order-disorder transformations, B2 to random bcc and random fcc to L1 2 , analyzed in the framework of the concentration wave-functional theory, appear to be ruled by the sizes of the relevant Fermi surface ͑FS͒ nesting vectors in the two random-alloy systems. The transformation between the bcc and fcc solid solution phase is studied by first-principles Korringa-Kohn-Rostoker and coherent-potential approximation calculations, and it is shown to be influenced by electronic topological transitions occurring in the same systems. Our analysis points out the preeminent role played by the lattice-incommensurate FS nesting vectors that in both the random-alloy systems, give rise to frustrated concentration waves. The location of the B2-L1 2 coexistence at Tϭ0 appears to be determined by the condition that the concentration waves of the fcc and bcc random alloys are equally frustrated.

Local charge excesses in metallic alloys: A local-field coherent potential approximation theory

“…There are many reasons for such a fortune: as it has been already pointed out, the CPA allows for a very careful determination of spectral properties 2 and Fermi surfaces. 3 However, for the arguments discussed in the present paper, two other aspects of the CPA are particularly amenable: its simple and elegant formulation 4 in terms of the multiple scattering theory 5 and the fact that, since it clearly defines a homogeneous meanfield alloy, it constitutes a natural starting point for perturbative studies of the fluctuations. The last feature has allowed the development of successful methods for the study of phase equilibria 6 and magnetic phenomena 7 in metallic alloys and led to surprisingly accurate calculations of properties connected with typical Fermi liquid effects, such as spin 8 and concentration waves.…”

Fermi Surface Nesting in DisorderedCu1−xPdxAlloys

“…One system that has been extensively studied is Cu 12x Pd x (see [5] for phase diagram), where the nesting is thought to be particularly strong. Below T c ͑ϳ800 K͒, this system adopts either the B2 or L1 2 long-range ordered structure or one of a wealth of long period superstructures, depending on composition (see, for example, [6,7]).…”

First-principles theory of the temperature and compositional dependence of atomic short-range order in disordered Cu-Pd alloys

“…Full details of these studies of the Fermi surfaces can be found in Ref. 52 and a comparison with experiment in Ref. 20.…”