1306.2154 (T. L. Underwood et al.)

Charge distribution and disorder broadening in alloys: simple models    [PDF]

T. L. Underwood, R. J. Cole

In this paper we consider the charge-excess functional model (CEFM) for net atomic charges in alloys [E. Bruno et al., Phys. Rev. Lett. 91, 166401 (2003)]. We begin by deriving the CEFM energy function in order to elucidate the approximations which underpin this model. We thereafter consider the particular case of the CEFM in which the strength of the `local interactions' within all sites are the same. We show that the ground state charges of this model can be expressed in terms of charge transfer between all pairs of sites in the same manner as the optimized linear charge model [T.L. Underwood et al., Phys. Rev. B 79, 024203 (2009)]. Hence the model considered is the generalization of the optimized linear charge model for alloys containing more than two chemical species. We then determine the model's unknown `geometric factors' over a wide range of parameter space. These quantities are linked to the nature of charge screening in the model, and we illustrate that the screening becomes increasingly universal as the strength of the local interactions are increased. We then use the model to derive analytical expressions for various physical quantities, including the Madelung energy and the disorder broadening in the core electron binding energies. These expressions are applied to ternary random alloys, for which it is shown that the Madelung energy and magnitude of disorder broadening are maximized at the composition at which the two species with the largest `electronegativity difference' are equal, while the remaining species having a vanishing concentration. This result is somewhat counterintuitive with regards to the disorder broadening since it does not correspond to the composition with the highest entropy. Finally, the model is applied to CuPd and CuZn random alloys. The model is ...

View original: http://arxiv.org/abs/1306.2154