On a surprising anisotropy of surface segregation in CuPt alloys

We present a theoretical study of the orientation dependence of surface segregation in CuPt alloys. A very unusual anisotropy is found: namely, the Cu surface enrichment is stronger for the close-packed (111) face than for the more open (100) one, contrary to usual arguments based on the anisotropy of surface tensions. This has to be related to the peculiarity of the ordering in CuPt alloys which, for equi-atomic concentration, tends to form at low temperature (in the ordered state) an alternate stacking of pure (111) planes, due to the existence of second-neighbour pair interactions larger than the first-neighbour ones. Therefore, the original surface segregation anisotropy found even in the disordered state can be explained by the synergy between surface tension and alloying effect in the (111) direction (the Cu surface plane being the termination of the ordered structure at low temperature) and the competition between the two phenomena in the (100) direction (a mixed CuPt plane being the terminal plane of the ordered structure at low temperature).