Oscillatory segregation at a metal alloy surface: Relation to ordered bulk phases

Some transition-metal alloys exhibit a complex surface segregation, where the composition oscillates with depth. Here, such behavior is predicted to occur for CuAu(100), based on Monte Carlo calculations using an empirical classical potential for multicomponent systems. A simple model shows that this behavior is related to the existence of an ordered phase in the bulk phase diagram, with the depth of the segregation diverging at the bulk ordering temperature. This accords with the observed behavior of PtNi, which has a phase diagram similar to CuAu. The role of surface orientation is discussed, as is the related nonoscillatory surface segregation of alloys exhibiting a bulk miscibility gap.