ANISOTROPIC CLUSTER MODEL FOR THE SHORT-RANGE ORDER IN CU1-XPDX-TYPE ALLOYS

The split diffuse maxima around the {110} and {100} positions in the d iffraction pattern of short-range-ordered Cu1-xPdx alloys (x=0.10...0. 60) are attributed to small atomic clusters, being part of the underly ing fee lattice. By analyzing the reciprocal space geometry, our clust er method identifies two prominent cluster types: the tetrahedron of n earest neighbors and a linear three-points cluster along the [110] dir ections. Since both cluster types contain different information on the same nearest-neighbor correlations, local anisotropy has to be assume d. It is shown that the three interatomic pair interactions within the se 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.