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

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 fee geometrical arrangements. We show how the phase equilibria between these intermetallic compounds can be understood by introducing a hypothetical bce random-alloy phase and a three-step transformation from B2 to random bce to random fee to L12. 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 sires of the relevant Fermi surface (FS) nesting vectors in the two random-alloy systems. The transformation between the bce and fee 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-L 1(2) coexistence at T = 0 appears to be determined by the condition that the concentration waves of the fcc and bce random alloys are equally frustrated.