Effect of chemical-potential fluctuations on short-range order in disordered alloys

By taking into account the chemical-potential fluctuations, we propose a generalization of the formalism elaborated by R V Chepulskii and V N Bugaev (1998 J. Phys.: Condens. Matter 10 7309), which is based on the use of the thermodynamic fluctuation method in the first order of a modified thermodynamic perturbation theory under the choice of the inverse effective number of atoms interacting with one fixed atom as a small parameter of expansion. As a result, a new approximation for calculation of the short-range order parameters in the disordered (i.e. without the long-range order) state of alloys is obtained. Moreover, the rigorous equation for determination of the chemical potentials is derived to replace the heuristic one obtained earlier. Both analytical and numerical comparison of the results of old and new approximations is performed. By comparison with the results of the Monte Carlo simulation, the higher numerical accuracy of the new approximation is demonstrated. The importance of taking into account the chemical-potential fluctuations is shown at low temperatures.