Influence of Impurity Charge-State on the Temperature Dependence of the Electric-Field Gradient

We present, for the first time, clear experimental evidence of the influence of the electronic configuration of probe-atoms on the temperature dependence of the electric-field gradient (EFG) tensor at impurity sites. We measured the EFG at 111 In →111 Cd and 181 Hf → 181 Ta impurity sites in a complete series of semiconducting and insulating compounds with the same crystalline structure, using the Perturbed-Angular-Correlation technique. The results of this systematic study show that while the principal component VZZ of the EFG increases with temperature at Cd-impurity sites, it decreases at Ta-impurity sites. This temperature dependence is associated with the redistribution of the electronic charge lack or excess located at the impurity center. A simple model is presented based on charge transfer from the probe to its neighbors, in terms of the acceptor and donor nature of the mentioned impurities.