Bulk ordering and surface segregation in ${\mathrm{Ni}}_{50}{\mathrm{Pt}}_{50}$

Interatomic interactions obtained from the effective screened generalized-perturbation method have been applied in Monte Carlo simulations to derive the bulk and surface-alloy configurations for ${\mathrm{Ni}}_{50}{\mathrm{Pt}}_{50}.$ The calculated order-disorder transition temperature and short-range order parameters in the bulk compare well with experimental data. The surface-alloy compositions for the (111) and (110) facets above the ordering transition temperature are also found to be in a good agreement with experiments. It is demonstrated that the segregation profile at the (110) surface of NiPt is mainly caused by the unusually strong segregation of Pt into the second layer and the interlayer ordering due to large chemical nearest-neighbor interactions.