The concentration-dependent position of the diffuse peaks in electron and x-ray diffraction patterns of Cu1-xPdx alloys in the disordered state is attributed to Fermi surface nesting. We present the first experimentally determined Fermi surfaces of Cu0.72Pd0.28 and Cu0.6Pd0.4, and show that they do indeed possess significant flat areas capable of nesting. Moreover, the magnitudes of the nesting vectors are in excellent agreement with those deduced from electronic structure calculations and diffraction experiments.

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.

Thermoreflectance measurements on NbxMo1−x alloys (x=0.2, 0.5, 0.8) have been carried out in the 0.5-5.0 eV energy region. Augmented-plane-wave (APW) calculations for Nb at two different lattice parameters and for Mo, as well as coherent-potential-approximation calculations (CPA), have been carried out and have been used in the interpretation of the experimental results. Several optical transitions [Σ1(EF)→Σ3, G4(EF)→G1,N2→N′1], have been identified, and their concentration dependence followed. These results contribute significantly toward putting the interpretation of the optical properties of Nb, Mo, and their alloys on a much more secure footing. In particular, it has been confirmed that while the lower conduction bands behave roughly as predicted by the rigid-band model, the higher-lying conduction bands show distinctly nonrigid-band-like behavior. KeywordsAmes Laboratory, augmented-plane-wave, rigid-band

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Soft X-ray spectra (SXS) a r e largely used to probe the band structure of metals and alloys /l/. Recently a number of standard band calculations have been