X-ray study of the ordering phase transition at the Cu3Au (111) surface

We have studied the phase transition at the (111) surface of Cu3Au by glancing incidence x-ray scattering. The experiment was designed to approach the general question of surface behavior near a bulk first order phase transition. We have studied three aspects of the phase transition: the surface order parameters, the order parameter fluctuations and the ordering kinetics. We found that close to the transition temperature, the surface ordering is inhomogeneous. Small clusters coexist with large ordered antiphase domains in the surface region. The number of the clusters increases with increasing temperature. These small clusters can be either disordered clusters or small antiphase domains. We suggest that the formation of small clusters is related to the heterophase fluctuations. The surface order parameter measurement has two possible explanations. The first possibility is that the surface order parameters approach zero continuously with temperature. The second possibility is that the surface order parameters have a finite jump at the transition point, while the number of small clusters increases with temperature so that the total surface area which are not occupied by the clusters approach zero continuously. The ordering kinetics at the surface has some of the same feature as the bulk. After the sample is quenched, small ordered domains nucleate at the surface. However, the coarsening of antiphase domains originates only deep in the bulk and propagates to the surface. We suggest that the phase transition at the (111) surface of Cu3Au IS a first order transition with strong heterophase fluctuations.U of I OnlyThesi