Predicting the segregation profile of the Pt25Rh75(100) surface from first-principles.

The segregation profile of the Pt(25)Rh(75)(100) surface is studied by the combination of density functional theory calculations with the cluster-expansion method and Monte Carlo simulations. We construct the stability diagram for the surface layers, which allows the prediction of the most stable atomic configuration for a given average concentration in those layers. On this basis, we apply the cluster-expansion Hamiltonian in grand-canonical Monte Carlo simulations for the prediction of the temperature-dependent concentration profile.

First-principles-based surface phase diagram of fully relaxed binary alloy surfaces.

The combination of density-functional theory (DFT) calculations of geometrically fully relaxed binary alloy surfaces with concepts from statistical physics is applied to construct a DFT-based phase diagram for a binary alloy surface. As a first example, we studied the appearance of Co antisite atoms at CoAl(100) surfaces. The structural parameters as multilayer relaxations, surface buckling, lateral order, and segregation profile of the predicted stable surface phases are in excellent agreement with experimental structure determinations applying low-energy electron diffraction.

Segregation in strongly ordering compounds: a key role of constitutional defects.

For the example of the B2 CoAl(100) surface, we demonstrate that even slight deviations from an ordered alloy's ideal stoichiometry in a subsurface region or in the bulk can drastically affect its surface composition. By experimental surface analysis and first-principles calculations, we show that Co antisite atoms segregate to the very surface, driven by the same strong interactions which enforce order in the bulk. Our findings are consistent with the lack of antisite segregation we found earlier for the much weaker ordering FeAl(100), and resolve contradictory reports for NiAl(100).