Calculated orientation dependence of surface segregations inPt50Ni50

We present local-density calculations of surface segregation profiles in a random ${\mathrm{Pt}}_{50}$ ${\mathrm{Ni}}_{50}$ alloy. We find that the concentration profiles of the three low-index surfaces oscillate and that the two most closely packed surfaces, i.e., (111) and (100), are enriched by Pt while Ni is found to segregate to the surface of the (110) face. These results may be explained in terms of a competition between the tendency of Pt to segregate towards the two first surface layers at the (110) face and the tendency to form a structure of alternating Pt and Ni layers.