Multi-site correlations and the atomic size effect

Optical diffraction patterns are presented which illustrate the effect of multi-site correlations on the diffraction pattern of a model disordered alloy consisting of different sized atoms placed on a simple square lattice. Monte Carlo methods were used to apply size-effect distortions to a number of example lattices containing various multi-site correlations. The presence of a four-site correlation between the four sites of the unit square resulted in correlations between the atomic displacements transverse to the correlation direction, and this resulted in clearly discernible extra diffuse peaks. Even though the magnitude of the four-site correlation was close to the physical limit for such a term the effect on the diffraction pattern was relatively small, the major component still being the sinusoidal modulations of the simple `size effect' which depend purely on the two-site correlations. The introduction of a three-site correlation between three of the four sites of the unit square, on the other hand, resulted in a substantial contribution to the scattering in the form of additional sinusoidal modulations along a diagonal direction.