Electronic theory of phase stability in substitutional alloys: application to the Au–Ni system

The total energies of formation of Au–Ni superstructures based on a fcc lattice have been calculated using the linear muffin-tinorbital (LMTO) method in the full potential approach. Both unrelaxed and relaxed structures have been included in the calculations. The energy of formation is decomposed into three terms, a volume deformation contribution, a chemical contribution and a relaxation contribution. The enthalpy of mixing of the disordered solid solution has been calculated using an Ising-like cluster expansion for both the chemical and relaxation effects. In the Gibbs energy of mixing, a configurational entropy of mixing has been calculated with the cluster variation method (CVM) and the thermal excitations due to electronic and vibrational effects have been taken into account. The miscibility gap displayed in the Au–Ni system has been calculated. The results are discussed in comparison with the experimental data.