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Phys. Rev. B 50 (1994) pp. 18564-18571
Effective cluster interactions at alloy surfaces and charge self-consistency: Surface segregation in Ni-10 at.% Al and Cu-Ni
Laboratorium für Festkörperphysik, ETH-Hönggerberg, CH-8093 Zürich, Switzerland
Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, United Kingdom
Abstract
First principles results are presented for the effective cluster
interactions at the surface of a random Ni-10 at.% Al alloy. The
derivation is based on an extension of the generalized perturbation
method to semi-infinite inhomogeneous binary alloys, using a layer
version of the Korringa Kohn Rostocker multiple scattering approach in
conjunction with the single site coherent potential approximation to
compute the self-consistent electronic structure of the system. When
applied to the bulk, the method yields effective pair interactions
which have the full point group symmetry of the lattice to a very high
level of numerical accuracy, despite of the fact that intra- and
interlayer couplings (scattering path operators) are treated
differently, and which are in perfect agreement with those of a recent
3-dimensional treatment. Besides the pair terms, a selected class of
triplet and quadruplet interactions are calculated, as well as the
point interactions induced by the presence of the surface. The value
of the latter in the first lattice plane is strongly exaggerated in
our approach, leading to a complete segregation of the minority
species to the surface. Using a value corresponding to the difference
in the surface energies of the pure components for this term leads to
the observed Al concentration of approximately 25% at the surface.
Possible reasons for the shortcoming of the theory are analyzed, and
test calculations for the well studied Cu-Ni system show that the free
energy of the seminfinite alloy cannot be approximated by the sum over
the single particle band energies, once charge selfconsistency is
enforced at the surface.