The split diffuse maxima around the {110} and {100} positions in the diffraction pattern of short-range-ordered ${\mathrm{Cu}}_{1-x}{\mathrm{Pd}}_x$ alloys $(x=0.10\mathrm{}\dots 0.60)$ are attributed to small atomic clusters, being part of the underlying fcc lattice. By analyzing the reciprocal space geometry, our cluster method identifies two prominent cluster types: the tetrahedron of nearest neighbors and a linear three-points cluster along the 〈110〉 directions. Since both cluster types contain different information on the same nearest-neighbor correlations, local anisotropy has to be assumed. It is shown that the three interatomic pair interactions within these basic clusters are sufficient to generate the spot splitting in the diffraction pattern. A ground-state analysis with these interactions reproduces the results of the anisotropic next-nearest-neighbor Ising model.

• Received 4 August 1997

DOI:https://doi.org/10.1103/PhysRevB.57.11132