FP-LAPW study of the EFG at impurity sites in oxides: Cd in rutile TiO2

We report here first-principles determination of the electric-field gradien t (EFG) tensor at the Cd impurity located at cation sites in rutile TiO2. A s far as we know, these represent the first ab initio calculations at impur ity sites with the FP-LAPW method in oxide systems. We used super-cells to simulate the diluted impurity in the crystal. The free-relaxation process p erformed in our study shows that the changes in distances of the oxygen nea rest-neighbours to the impurity are not isotropic as was supposed in a prev ious study within the muffin-tin approximation. Our prediction for the EFG component of major absolute value agrees well with the experiment and is op posite in sign and direction if isotropic relaxations are assumed. Our valu e of the asymmetry parameter eta also compares very well with the experimen tal value.