Order-disorder transitions at and segregation to (001) Ni-Pt surfaces

Order-disorder transitions at and segregation to the (001) surface of Ni-Pt alioys have been investigated by a recently developed free energy simulation method, where the atomic interactions are described using the embedded atom method (EAM) potentials. On the Ni-rich side of the phase diagram, we observe a second order, order-disorder phase transition on the (001) surfaces at temperatures well above the bulk phase transition temperature. At the transition temperature, the first (002) atomic plane changes from a disordered plane to an ordered one with the ~(2 X 21 pattern. The second (002) plane changes from a disordered plane to a nearly pure Ni plane. Subsequent planes retain their essentially bulk-like, disordered structure. We also observe first order, order-disorder surface phase transitions on the P&rich side of the phase diagram. At the transition temperatures, the first and third (0021 planes become nearly pure Pt and the second plane becomes nearly pure Ni. The effect of the surface transitions on the thermodynamic properties of the surfaces, such as enthalpy and vibrational and configurational entropy, are also investigated. It is shown that the ordering surface transition upon cooling the Ni-rich sample is enthalpically driven. The present simulations also show the importance of including atomic vibrations in surface segregation studies. Atomic vibrations have been typically omitted in previous lattice gas descriptions of surface segregation.