Electric-field gradients at 181Ta impurity sites in Ho2O3 and Eu2O3 bixbyites

The time-differential γ–γ perturbed-angular-correlation (PAC) technique with ion-implanted 181Hf tracers has been applied to study the hyperfine interactions of 181Ta impurities in the cubic bixbyite structure of Ho2O3 and Eu2O3. The PAC experiments were performed in air in the temperature range 300–1373 K (in the case of Ho2O3) and 77–1273 K (in the case of Eu2O3). For both oxides, two electric-quadrupole interactions were found and attributed to the electric-field gradients (EFGs) acting on 181Ta probes substitutionally located at the two free-of-defects nonequivalent cation sites of the bixbyite structure. In the case of Ho2O3, two additional interactions were found in the temperature range 300–573 K. These results, as well as previous characterizations of the EFG at 181Ta sites in bixbyites, were compared to those obtained in experiments using 111Cd as probe, and to point-charge model calculations. Very recent ab initio predictions for the EFG tensor at impurities sites in binary oxides are also discussed. All these results enable us to discuss the validity of the widely used ionic model to describe the EFG in these highly ionic compounds.