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 inf luence of the electronic configuration of probe-atoms on the temperatu re dependence of the electric-held gradient (EFG) tensor at impurity s ites. We measured the EFG at In-111 --> Cd-111 and Hf-181 --> Ta-181 i mpurity sites in a, complete series of semiconducting and insulating c ompounds with the same crystalline structure, using the Perturbed-Angu lar-Correlation technique. The results of this systematic study show t hat while the principal component V-ZZ of the EFG increases with tempe rature at Cd-impurity sites, it decreases at Ta-impurity sites. This t emperature dependence is associated with the redistribution of the ele ctronic charge lack or excess located at the impurity center. A simple model is presented based on charge transfer from the probe to its nei ghbors, in terms of the accepter and donor nature of the mentioned imp urities.