Impurity-impurity interaction energies in Cu, Ni, Ag, and Pd and fundamental features of phase diagrams of binary alloys and solid solubility limit of impurities: KKR-Green's function method and cluster variation method

The impurity-impurity (X-X; X=Ti-Cu, Zr-Ag) interaction energies, from Ist- neighbor to 8th-beighbor, in metals (Cu, Ni, Ag, Pd),are calculated as accu rately as the lattice parameters and bulk moduli of complete metals [Materi a Japan, 37(1998), 564], based on the Korringa-Kohn-Rostoker Green's functi on method for point defects [in Computational Physics as a New Frontier in Condensed Matter Research. ed. by H. Takayama et al., Physical Society of J apan, Tokyo, (1995), 105-113], using density functional theory. We use the local-spin-density approximation (LSDA) and the generalized-gradient approx imation (GGA) of Perdew and Wang [in Electronic Structure of Solids '91 ed. by P. Zieche et al., Academic Verlag, Berlin, (1991), 11-20]. First we rev iew the present status of the first-principles calculations for metals; the GGA calculations correct very well the deficiencies of the LSDA for metals , i.e, the underestimation of equilibrium lattice parameters and the overes timation of bulk moduli. Secondly we show that the fundamental differences among segregation, solid solution, and ordering behavior of the binary allo ys of impurity and host elements considered here, known experimentally, may be distinguished very well by use of the present impurity-impurity interac tion energies. It should be noted that the observed ordering behavior of Ni X (X=Mn, Fe, Co) may not be reproduced without the spin-polarization effect . The importance of magnetism in Ni-based alloys is quantitatively discusse d by comparing with the calculated results for non-magnetic Pd-based alloys . Finally we show that the temperature dependence for the solid solubility limit of impurities in metals, such as Rh in Pd and Ru in Pd (PdRh and PdRu are segregated at low temperatures and become disordered at high temperatu res), may be reproduced very accurately by the cluster variation method bas ed on the present impurity-impurity interaction energies. We also found tha t the inclusion of the far-neighbor interaction energies is very important.