Surface phase stability diagram for Pd deposits on Ni(110): A first-principles theoretical study

Different surface structures of Pd deposits on Ni(110), from alloys to ordered phases, in a range between 0.125 monolayer and near 4 monolayers, have been studied using density functional calculations, The main feature of these deposits is the stress exerted by the Ni substrate on the Pd atoms, due to the size difference between the two metals. The surface phase stability diagram has been constructed, allowing to find the most stable surface structures at a given coverage. At low coverage (less than or equal to0.7 monolayer), the disordered alloy phase is found to be stable. Then, at higher coverage, ordered surface structures are stable, and a mixing of some or them can coexist in some ranges. Between 0.7 and 1 monolayer, the surface presents ail equilibrium between the alloy phase (containing 70% of Pd atoms) and a 1 monolayer phase. In the 1-4 monolayers range, the equilibrium is set up between the two extreme phases: 1 monolayer and 4 monolayers: These deposits show a peculiar structure, in which a periodical Pd vacancy at the Ni-Pd interface ensures the release of the stress and induces a heteroepitaxial dislocation. The stability of these structures versus the coverage is discussed as a competition between surface alloying and surface stress. Finally, the electronic structure of these phases is studied and appears to be closely linked to the surface strain of the Pd deposit, even for the alloy phases. These bimetallic surfaces reveal a wide range of local density of states, simply by changing the Pd coverage. Then, the possibility to tune the specific catalytic properties of these deposits is discussed.