Distribution of electric field gradients around impurities in aluminum

The electric field gradients caused by a vacancy, Mg, Ga, In, Si, Ge and Sn impurities at several near-neighbor sites in Al hosts have been calculated. The theory takes into account the contribution from the conduction electron screening cloud and the lattice strain caused by the impurities. The perturbed core as well as conduction electron densities around the impurities are calculated selfconsistently using the density functional theory. Assuming that the charge distribution around the impurity is spherically symmetric, an exact expression, valid at all distances, is derived for the conduction electron contribution to the electric field gradient. While this result is substantially different from those using the conventional asymptotic or pre-asymptotic expressions, it is found to be entirely inadequate in explaining the observed asymmetry and magnitude of the electric field gradient distribution in cubic metal alloys. The contribution due to the lattice strain is calculated using the point-ion model and a new analytic form for the elastic strain tensor. The combined strain and charge screening effect provides a satisfactory agreement between calculated and experimental electric field gradients. The difficulties standing in the way of an overall quantitative understanding of the electric field gradient in cubic metal alloys are discussed. The subsequent stages of improvement in both theory and experiment that can result in a better understanding of the problem are pointed out.