Broken bond model alloys

Simple criteria for surface segregation in alloys (relative melting points, enthalpies of sublimation, metal atom radii, surface free energies of the pure metals) all indicate that surface segregation of titanium should occur on Pt/Ti alloys in vacuo. However, this is inadequate because of the large departures from ideality in Pt/Ti alloys. Analysis (11) of a broken bond model of the system, especially with the use of data directly determined with Pt/Ti alloys, gives a more reliable result. 

The theoretical analysis (11) of surface segregation in PtjTi, on the same basis as the analysis of dilute solid solutions, predicts no surface segregation at exact stoichiometry. The broken bond model of surface segregation is also successful in the prediction of surface segregation in some other non-ideal platinum alloys (17). 

Although the discussion on the broken bond model has been mainly focused on pure metallic surfaces, the extension can be easily made simply by replacing the binding energy term Ej, in Eq. (1) with the interaction energies among different atoms in the case of metallic alloys [15-17]. However, in the case of ceramic materials the... 

The main handicap of MD is the knowledge of the function [/( ). There are some systems where reliable approximations to the true (7( r, ) are available. This is, for example, the case of ionic oxides. (7( rJ) is in such a case made of coulombic (pairwise) interactions and short-range terms. A second example is a closed-shell molecular system. In this case the interaction potentials are separated into intraatomic and interatomic parts. A third type of physical system for which suitable approaches to [/( r, ) exist are the transition metals and their alloys. To this class of models belong the glue model and the embedded atom method. Systems where chemical bonds of molecules are broken or created are much more difficult to describe, since the only way to get a proper description of a reaction all the way between reactant and products would be to solve the quantum-mechanical problem at each step of the reaction. 

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