Properties ordered surface alloy

In the quest for the highest selectivity and improved performance of heterogeneous catalysts, new fundamental information on the reactivity and properties of bimetallic alloys win be helpful. The weU-defmed structures of ordered surface alloys (intermetallic... 

The BFS method has been applied to a variety of problems, ranging from the determination of bulk properties of solid solution fee and bee alloys and the defeet strueture in ordered bee alloys [28] to more speeifie applieations ineluding detailed studies of the strueture and eomposition of alloy surfaees [29], ternary [30] and quaternary alloy surfaees and bulk alloys [31,32], and even the determination of the phase strueture of a 5-element alloy [33]. Previous appheations have foeused on fundamental features in monatomie [26] and alloy surfaces [29] surface energies, reconstructions, surface structure and surface segregation in binary and higher order alloys [34,35] and multilayer relaxations [36,37]. While most of the work deals with metallic systems, the lack of restrictions on the type of system that can be studied translated into the extension of BFS to the study of semiconductors [38]. 

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... 

The term surface alloy is somewhat generic and may refer to a variety of different systems. Here, we apply it to those systems where ultra-thin metal layers (i.e. a few atomic layers thick) are deposited on a bulk metal surface and where the system is subsequently annealed in vacuum in order to obtain alloying in a surface region a few atoms thick. In these conditions it is possible to obtain single atomic layer binary phases, or multilayer surface alloy phases (also termed epitaxial alloys (for a general discussion of these surface alloys, see [5]. Relatively to the subject of the present paper, two Pt-Sn systems have been studied Sn-Pt(l 11) and Sn-Pt(lOO). The behavior and the structural properties of these systems will be discussed in detail in the following. 

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. 

From all that was said above, it follows that the polymer alloy is a comph-cated midtiphase system with properties which are determined by the properties of constituent phases. It is very important to note that if, on the macrolevel, the thickness of the interphase regions is low, as compared with the size of the polymer species, for small sizes of the microregions of phase separation such approximation is not vahd. In comparison with the size of the microphase regions, the thickness of the interphase may be of the same order of magnitude. Therefore, they should be taken into accoiuit as an independent quasi-phase in calculation of properties of polymer alloys. We say quasi-phase because these region are not at equilibrium and are formed as a result of the non-equilibrium, incomplete phase separation. The interphase region may be considered as a dissipative structure, formed in the coiu-se of the phase separation. Although it is impossible to locate its position in the space (the result of arbitrary choice of the manner of its definition), its representation as an independent phase is convenient for model calculations (compare the situation with calculations of the properties of filled polymer systems, which takes into account the existence of the surface layer). 

Unfortunately, SRO is temperature dependent Consequently, temperature becomes an extremely important parameter in surface segregation. We will see that in experiment the annealing temperature chosen may be essential for the resulting properties. For some alloys, the annealing temperature necessary to reach a thermodynamical equilibrium lies far beyond the melting temperature. So if we compare measured and predicted data, one has to make sure that parameters such as temperatures are treated correctly in order not to compare apples and oranges. 

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