Ordering and segregation in terms of the polar model of an alloy

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Abstract
In earlier work, Mott (1937) showed that the electrostatic energy of CuZn could account for most of the ordering energy. Harrison and Paskin (1962) have subsequently found that the introduction of more recent concepts of electron screening in metals when applied to AB alloys leads to a polar model resembling that suggested by Mott (1937). We show here that the polar contribution calculated by following the procedure of Mott (1937), always leads to ordering whereas the Harrison and Paskin (1962) formulation can account for ordering or segregation. In both polar calculations (Mott 1937 and Harrison and Paskin 1962), the origin of the ordering energy is the incomplete (or excessive) screening of the A and B ionic charges. We denote the relevant charges in an atomic polyhedron surrounding an x-type of atom as Qx = Qx i—Qx e, where Qx i equals the ionic charge of x minus the average number of conduction electrons per atom and – Qx e is the electronic charge screening Qx i. Mott (1937) makes an electrostatic calculation of E T the total energy of ordering as follows:

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