Electronic Properties of Alloys and Theoretical Models

The heart of the problem is this a copper atom in a nickel matrix does not wish to lose its 4s electron totally, nor does a nickel atom wish to accept, it as this would be tantamount to forming an ionic bond Ni Cu+. While electrostatic bonding can contribute to the stability of some intermetallic compounds, as in the 

Cs+Au type of compound, which we have already met, it is not a suitable basis for explaining the formation of alloys. An early attempt to solve at least part of the problem involved looking at a nickel atom in a copper matrix. The 3d electrons of the nickel atom occupied highly localised energy levels around it, but they were broadened by resonant interaction with the 4s electrons of copper. The width of the d-band should increase with nickel concentration as the d-electrons 

We now have to think how the chemical interaction of the components of the alloy when at the surface affects their ability in chemisorption. Before we can look at this, however, we must address the problem of surface segregation. 

The formalism used for liquid solutions can also be applied to solids For an ideal solution, the enthalpy of mixing is zero, and by application of the Gibbs equation we can deduce that the ratio of the mol fractions of the two components in the surface X /xi is given by 

When is positive, bonds between unlike atoms are preferred and greater enrichment occurs at low bulk concentrations, but if it is negative bonds between like atoms are preferred and greater enrichment occurs at high bulk concentrations. The somewhat complex equations which describe surface enrichment for real solutions, i.e. when is not zero, can also predict concentration differences in second, third and fourth layers. 

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