Madelung potentials in disordered systems

We analyse the contribution of Madelung terms to the local potential on atomic sites in disordered alloys, and assess the extent to which the variation in the local potential should be observable in high-resolution x-ray photoelectron experiments. We show that a model in which the charge on an atom is determined by the global composition predicts considerable broadening of core-level photoelectron spectra. Such disorder broadening has not been observed experimentally. A model in which the local charge is determined by the number of `unlike' nearest neighbours predicts a much smaller disorder broadening, but one which should be observable in high-resolution measurements for some alloy systems.