First-principles study of short range order and instabilities in AuCu, AuAg and AuPd alloys

Interaction energies derived from first-principles total energy calculation are used together with the Cluster Variation Method in order to study phase diagrams, ordering instabilities and the Fourier spectrum of the Warren-Cowley short range order parameters for AuCu, AuAg and AuPd alloys. The present study is carried out using both the tetrahedron and the tetrahedron-octahedron approximations of the CVM. It is shown that the tetrahedron approximation predicts the wrong Fourier spectrum for the SRO parameters although it gives qualitatively correct phase diagrams. In particular, the tetrahedron approximation fails to reproduce the expected maxima in the fluctuation spectrum at the 〈1,0,0〉-type reciprocal space vectors, characteristics of L1₀ and L1₂ ordering systems. The correct features of the k-space fluctuation spectrum is obtained in the tetrahedron-octahedron approximation.