ORDER-DISORDER TRANSITIONS AT AND SEGREGATION TO (001) NI-PT SURFACES

Order-disorder transitions at and segregation to the (001) surface of Ni-Pt alloys have been investigated by a recently developed free energ y 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 transi tion on the (001) surfaces at temperatures well above the bulk phase t ransition temperature. At the transition temperature, the first (002) atomic plane changes from a disordered plane to an ordered one with th e c(2 x 2) pattern. The second (002) plane changes from a disordered p lane to a nearly pure Ni plane. Subsequent planes retain their essenti ally bulk-like, disordered structure. We also observe first order, ord er-disorder surface phase transitions on the Pt-rich side of the phase diagram. At the transition temperatures, the first and third (002) pl anes become nearly pure Pt and the second plane becomes nearly pure Ni . The effect of the surface transitions on the thermodynamic propertie s of the surfaces, such as enthalpy and vibrational and configurationa l entropy, are also investigated. It is shown that the ordering surfac e transition upon cooling the Ni-rich sample is enthalpically driven. The present simulations also show the importance of including atomic v ibrations in surface segregation studies. Atomic vibrations have been typically omitted in previous lattice gas descriptions of surface segr egation.