Calculation of electric-field gradient in a dilute alloy of Al-Mg

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Abstract
The existing calculations of electric-field gradient (EFG) in dilute Al-based alloys with nonmagnetic substitutional impurities are unsatisfactory on two accounts. First, the magnitudes of the calculated EFG are very different from the experimental results. Second, these calculations predict an axially symmetric EFG at the nearest-neighbor host sites from the impurity ion, contrary to experimental observations. The discrepancy between experiments and these calculations is largely due to the neglect of the size-effect EFG in the latter. In the present paper, we have calculated in the case of Al-Mg alloy contributions to EFG from both the valence effect and the size effect. The EFG from the valence effect has been calculated from a free-electron-screening charge distribution assumed for the alloy. The size-effect EFG tensor, linear in local strain around the impurity, has been evaluated in the elastic continuum approximation. The present result not only provides improved values for the magnitudes of EFG but also predicts a nonzero asymmetry parameter at the nearest-neighbor host sites from the impurity ion, in fair agreement with experiment.