Local-environment effects on the density of states and substitutional impurities in random alloys

We study the electronic structure of a binary alloy using the effective-medium approximation (EMA) and we include the local environment by treating the system as a multicomponent alloy with different neighbor compositions. We use a tight-binding model in the nearest-neighbor approximation. On introducing an impurity a spectrum of levels is found. A simplified EMA which includes the local environment is developed which is as easy to evaluate as the coherent-potential approximation. By making a perturbation about the virtual-crystal approximation a simple scheme which models the splitting of impurity levels is found. We extend the EMA to consider the effects of short-range order with nonrandom correlations and find that our approximations reduce to the exact results in the appropriate limits.