Ordering and segregation processes in transition metal alloys in relation to their electronic structures

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Abstract
Ordering and segregation processes in binary alloys and compounds are usually studied by using phenomenological pair interactions. Such a model can be justified when the perturbation expansion as a function of the difference between the ionic potentials of the alloy components is rapidly converging. This is not the case when dealing with many transition metal alloys especially when the components are rather far from each other in the periodic table. In this paper it is shown that it is still possible in this case to define effective pair interactions for ordering processes even if the total energy cannot be written in this way. We calculate the electronic contribution K(q) to the free energy variation induced by a concentration fluctuation with wave vector q. To this end, we use an extension of the coherent potential approximation to inhomogeneous systems and we keep only the first terms in an expansion as a function of the local concentration fluctuations. Applications are given concerning segregation processes (long-wave limit; spinodal decomposition) and ordering processes : particular attention is paid to the CsCl transitional compounds such as TiFe, TiCo, etc.