The authors propose a very simple method of calculating short-range order parameters alpha ij in disordered alloys using a series expansion in powers of a parameter gamma =exp(-1/ xi ) where xi is the dimensionless correlation length of the pair correlation function. In authors' approximation the sum rule alpha ii=1 is satisfied exactly, unlike in previous theories. In the zeroth order their approach leads to the spherical model results. The high accuracy of the theory developed is illustrated by comparing its results with those of Monte Carlo simulation and estimating the pair interactions from diffuse scattering data. In the latter problem pair-interaction potentials are explicitly expressed in terms of experimentally determined quantities.
The theory of short-range order in alloys developed previously by the present authors is used to resolve the ambiguities existing in the problem of determination of pair potentials from diffuse-scattering measurements.
The authors develop a simple method to calculate short-range order and temperatures of order-disorder transitions in binary alloys using a series expansion in powers of a parameter gamma =exp(-1/ xi ), where xi is the dimensionless correlation length of the one-electron Green function. In the zeroth order their approach gives the theory of Ducastelle and Treglia (1980) (coherent-potential and Bragg-Williams approximations). In the lowest non-trivial order they obtain the spherical model results for transition temperatures and correlations in the disorder phase, with the effective nearest neighbour interaction potential identical to that of the generalised perturbation method. It turns out that in this order the corrections to the zeroth-order electronic Green function can be neglected, so the authors have the calculation scheme based on the well-known coherent-potential approximation.
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