Monte Carlo simulation of Cu segregation and ordering at the (110) surface of Cu75Pd25

The segregation to the (110) surface of a Cu75Pd25 single crystal is modelled as a function of temperature by Monte Carlo simulations combined with the embedded atom method (EAM). Using EAM parameters from literature, optimised for the six transition metals of the Cu and Ni groups, does not yield satisfactory results: too low a bulk order disorder transition temperature T-c and, contrary to experimental evidence, Pd surface segregation and a disordered surface layer are obtained. These observations are contradicted by the LEIS results of Bergmans et al. [Surf. Sci. 345 (1996) 303] that revealed an ordered bulk with an oscillating concentration profile at least up to 600 K and a Cu-enriched surfacer They also observed a (2 x 1)LEED pattern caused by ordering in the second layer. In this work the EAM parameters are recalculated and optimised precisely for the Cu Pd alloy system under study. The results of the simulations are now in perfect agreement with the experimental findings: a bulk critical order-disorder temperature in agreement with the bulk Cu- Pd phase diagram, an oscillating concentration profile. Cu segregation to the surface and a second atomic layer exhibiting a substantial degree of ordering that accounts for the observed (2 x 1) LEED pattern. (C) 1998 Elsevier Science B.V. All rights reserved.