Atomic Arrangements in AuPt/Pt(100) and AuPd/Pd(100) Surface Alloys: A Monte Carlo Study Using First Principles-Based Cluster Expansions

We have constructed model Hamiltonians for AuPt/Pt(100) and AuPd/Pd(100) surface alloys based on the cluster expansion method and density functional theory. The cluster expansion Hamiltonians were used in Monte Carlo simulations to study the equilibrium arrangements of surface atoms in these two systems for a range of compositions and temperatures. We report on and explain results from these simulations in terms of the differing interatomic interactions present in each alloy. In AuPt surface alloys, homonuclear Pt-Pt interactions are favored over heteronuclear Au-Pt interactions, whereas in AuPd the opposite is true. Accordingly, our simulations show that Pt prefers to agglomerate, whereas Pd prefers to form smaller contiguous ensembles, such as monomers and dimers. Our simulations also reveal that the AuPd surface alloy can adopt c(2 x 2) ordering at low temperatures and 50% Pd coverage and exhibits a tendency for Pd monomers to occupy sites at the second nearest-neighbor distance from one another. Finally, we compare experimental data available in the literature to our results and find them in good qualitative agreement.