Segregation and ordering at alloys surfaces: modelling and experiment confronted

Abstract Model calculations combining the Monte Carlo (MC) method and a suitable energy model are proposed as fully complementary to experiments in order to gain insight in segregation and other surface phenomena, deployed by materials in order to minimise their surface free energy and hence their total Gibbs free energy. In the confrontation between experiments and modelling, it is basically assumed that the surface, as observed in the experiments, corresponds to the equilibrium situation of minimal Gibbs free energy. The entropy part is modelled by the stochastic nature of Monte Carlo simulations, while the energy part is taken into account by the (modified) embedded atom method ((M)EAM). Special attention is paid to the derivation of model parameters that are specific for the alloy under study. For it appears that in several cases the experimental data can only correctly be reproduced with these specific EAM parameters. This article further focuses on the determination of the degree of order in the bulk. Simulations according to the Grand Canonical Ensemble require the difference in chemical potential between the components. A novel approach is presented for deriving this parameter both for slightly off-stoichiometric ordered alloys and for disordered alloys. Results of MC/(M)EAM simulations are presented for the surfaces of five catalytically important binary alloys: Au 75 Pd 25 (1 1 0), Cu 75 Pd 25 (1 1 0), Pt 50 Ni 50 (1 0 0), (1 1 0) and (1 1 1), Pt 80 Fe 20 (1 1 1) and Pt 75 Sn 25 (1 1 1). It can be concluded that these simulations yield excellent predictions for surface modifications and are a very powerful tool to model and understand surfaces at equilibrium.