Surface alloy configuration

The local equilibrium surface alloy configuration and structure may be found by minimization of the surface free energy, or if several different phases may exist, by finding a convex hull of the lowest free energies of different phases at different alloy compositions (at T=0), or more generally by a common-tangent construction which is completely analogous to the usual treatment of the bulk systems. The procedure is illustrated in Fig. 4. 

The objective of this section is to introduce the BFS-based methodology for a detailed study of the most important features of surface alloy formation. The methodology assumes no a priori information on the system at hand. The only input necessary consists of the basic parameterization of the participating elements and lattice structures needed, as described in Sec. 2, and a catalogue of atomic distributions, where each configuration represents a state accessible by the system under study. Each entry in the catalogue is a computational cell popu-... 

The deposition of Au on Ni(llO) leads to the formation of a one-layer surface alloy [17], in spite of the fact that Au and Ni are immiscible metals. Starting with a single gold adatom deposited on a hollow site on the Ni(l 10) substrate, we consider five possible configurations, shown in Fig. 8. Table 3 displays results for the energy of formation per atom of each cell. These configurations include a Au atom located (a) in the overlayer (Au(0)), (b) in a substituted site in the surface plane (Au(S)), with the substituted Ni atom in the overlayer nearest-neighbor site (Au(S)Ni(0)i), (c) same, with the substituted Ni atom in the overlayer, far from the impurity (Au(S)Ni(0)f), (d) in the first plane below the surface (lb) with the displaced Ni atom somewhere in the overlayer, (Au(lb)+Ni(0)), and (e) two planes below the surface plane (Au(2b)+Ni(0)). The intermediate columns indi-... 

Fig. 15 Energy difference AE (in eV/atom) between unrelaxed and relaxed configurations of a Pd/Ni(l 10) surface alloy for 0.5 ML Pd coverage.

Difference in energy of formation AE (eV/atom) between the energy configuration with all the Au atoms forming an island in the overlayer and the configuration where Au and Cu form a surface alloy similar to the Q13AU 1 1 surface. 

In the case of Cu 100 -c(2x2)-Pd, ARUPS studies have identified a marked withdrawl of the Pd d-band from the Fermi level due to the absence of Pd-Pd nearest neighbour bonding [25,26,27]. The Pd atoms substituted within the copper surface appears to adopt a closed d-band electronic configuration, hence would be expected to have significantly different chemisorption and reactivity properties with respect to pure Pd surfaces. In agreement with its Cu 100 -c(2x2)-Au counterpart, the Pd d-band emission shows little or no dispersion as a function of photon energy in normal emission ARUPS, consistent with formation of a largely two-dimensionally confined surface alloy. 

In the following, surface and sub-surface alloy formation of ordered systems in ultra high vacuum will be discussed as an option to generate different surface configurations with dissimilar properties from the same set of material composition. Certainly, alloys develop also at the liquid-solid interface [10-13], yet the topic will not be covered in this chapter being of special devotion to ordering effects. Surfaces of ordered bulk alloys shall be reviewed in a first part. A second subdivision includes the formation of surface and subsurface alloys, whereas in a third section applications are discussed to grow ordered superstructures on top of alloy surfaces. 

As a matter of fact, opposed to solids with single elemental composition, the class of binary alloys obviously resembles an additional fi eedom to produce from the same bulk material various new surface configurations marked by different chemical reactive states and even completely new surface alloys. 

Langmuirian view, the active catalytic surface is comprised of a uniform distribution of static sites that do not interact with one another. This is sharply contrasted by the Taylor view, which proposes vacancies and topologically unique surface atom configurations as the centers of reactivity. The Langmuirian idea of a catalytically reactive surface leads to the ensemble effect that ascribes the changes in the selectivity for an alloy surface to the dilution of multi-atom surface ensembles in the alloy induced by mixing inert components into the active surface. In this view, the selectivity of a particular reaction depends predominantly on the number of reactive surface atoms that participate in elementary reaction events. 

Phase transitions in two-dimensional (adsorbed) layers have been reviewed. For the multicomponent Widom-Rowlinson model the minimum number of components was found that is necessary to stabilize the non-trivial crystal phase. The effect of elastic interaction on the structures of an alloy during the process of spinodal decomposition is analyzed and results in configurations similar to those found in experiments. Fluids and molecules adsorbed on substrate surfaces often have phase transitions at low temperatures where quantum effects have to be considered. Examples are layers of H2, D2, N2, and CO molecules on graphite substrates. We review the PIMC approach, to such phenomena, clarify certain experimentally observed anomahes in H2 and D2 layers and give predictions for the order of the N2 herringbone transition. Dynamical quantum phenomena in fluids are also analyzed via PIMC. Comparisons with the results of approximate analytical theories demonstrate the importance of the PIMC approach to phase transitions, where quantum effects play a role. 

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