Surface random alloys

I.A. Abrikosov and H.L.Skriver, Self-consistent linear-muffin-tin-orbitals coherent-potential technique for bulk and surfaces calculations Cu-Ni, Ag-Pd, and Au-Pt random alloys, Phys. Rev. B 47, 16 532 (1993). 

In the last decade an abundant literature has focused more and more on the properties of low-symmetry systems having large unit cells which render unwieldy the traditional description in terms of the Bloch theorem. Low-symmetry systems include compUcated ternary or quaternary compounds, man-made superlattices, intercalated materials, etc. The k-space picture becomes totally useless for higher degrees of disorder as exhibited by amorphous materials, microcrystallites, random alloys, phonon-induced disorder, surfaces, adsorbed atoms, chemisorption effects, and so on. 

Fig. 5. Surface energy curves for a monolayer of a random alloy on surfaces of pure metals.

Focusing our interest towards special sites available for adsorption, W g(Ar) for a nearest-neighbour vector Ar directly yields the fraction of (two-fold) bridge-sites between unlike atoms. Unfortunately, A(Ar) or W gCAr) do not directly yield the number of three-fold or four-fold hollow sites neighbouring, e.g., only atoms of type A. Nevertheless, we will see that the value of also gives an indication whether such sites are more or less common than in a random alloy surface. 

Turning to fcc(lll) surfaces, let us first compare the bulk properties of the alloys under consideration (Table 2). Stoichiometric PtFe, PtCo and PtNi form LIq phases in the bulk, characterised by an increase of the number of unlike nearest neighbours (8) with respect to an fee random alloy with 50% Pt (6 unlike NN). PtCu forms the peculiar LI, ordered phase, where the number... 

A full conversion from the c(4x4) to c(2x2) phase should result in 0.5 ML of displaced Cu. Previous STM measurements are consistent with this expectation [35]. However, LEEM measurements taken with the sample at 400 K, indicate that there is only 0.23 ML of added material during the conversion. To account for this difference, Plass and Kellogg [82] propose a new model for the higher temperature c(2x2) phase in which some of the Cu remains randomly alloyed in the c(2x2) overlayer structure. This model is at odds with LEED I-V analysis of Hosier and co-workers [79], who concluded that the c(2x2) structure is a pure overlayer phase. The discrepancy is still not resolved, but may be due to the different manner in which the c(2x2) phase was prepared in the two studies. In the LEED I-V study the overlayer was prepared by depositing excess Pb and flashing the surface repeatedly to 700 K until the c(2x2) pattern was its strongest. In the LEEM study the c(2x2) structure evolved from the c(4x4) structure during Pb deposition at 400 K. 

Figure 4.4 (a) Random alloy surface with two chemical... 

Figure 4.38 Equilibrium structure of the (100) surface of the random alloy Feo.97Alo.03 with the surface being c(2x2) reconstructed because of Al segregation to the surface (after [109]).

As we have seen, surface segregation in alloys is a fascinating and diverse effect It is not restricted to the surfaces of random alloys (solid solutions) and also occurs in... 

This appears as a random non-branching white tunnel of corrosion product either on the surface of non-protected metal or beneath thin surface coatings. It is a structurally insensitive form of corrosion which is more often detrimental to appearance than strength, although thin foil may be perforated and attack of thin clad sheet (as used in aircraft construction) may expose the less corrosion resistant aluminium alloy core. Filiform corrosion is not commonly experienced with aluminium, as reflected by the insignificance afforded it in reviews on the phenomena (Section 1.6). 

Electrochemical Properties All C Vs are presented on two different scales to show both the larger and smaller peaks in sufficient detail. At low Pt surface concentrations, the base CVs are very similar to those of the Pt island-modified Ru(OOOl) surfaces (see Fig. 14.5). With increasing Pt surface content, however, the charge in the Hupmore than one Ru atom were required for OHad and/or Hupd adsorption. Since the atom distribution in PcRui a /Ru(0001) surface alloys is very close to a random distribution [Hoster et al., 2008], the number of Ru sites is proportional to xj/u or (1 — xpt)". As is evident from the plot in Fig. 14.6, the experimental data agree very... 

The influence of Pt modihcations on the electrochemical and electrocatalytic properties of Ru(OOOl) electrodes has been investigated on structurally well-defined bimetallic PtRu surfaces. Two types of brmetalhc surfaces were considered Ru(OOOl) electrodes covered by monolayer Pt islands and monolayer PtRu/Ru(0001) surface alloys with a highly dispersed and almost random distribution of the respective surface atoms, with different Pt surface contents for both types of structures. The morphology of these surfaces differs significantly from that of brmetaUic PtRu surfaces prepared by electrochemical deposition of Pt on Ru(0001), where Pt predominantly exists in small multilayer islands. The electrochemical and electrocatal5d ic measurements, base CVs, and CO bulk oxidation under continuous electrolyte flow, led to the following conclusions ... 

---