SEGREGATION PROFILES IN CU3AU ABOVE THE ORDER-DISORDER TRANSITION

The segregation at a (100) surface of Cu3Au is studied slightly above the order-disorder transition at T-c. Based on a semi-infinite lattice-gas model with nearest- and next-nearest-neighbor interactions, we find within mean-field theory that close to the surface and in the disordered phase the Au concentration decays exponentially towards its bulk value. The corresponding decay length Lambda increases as (T-T-sp)(-1/2) upon approaching T-c, where T-sp < T-c denotes the spinodal temperature of the order-disorder transition. Without using any adjustable parameters, we obtain theoretical predictions in good quantitative agreement with recent x-ray-scattering data from this surface.