Electric-field gradients at 181Ta and 111Cd impurity sites in bixbyites

L. A. ERRICO, M. RENTERÍA, A. G. BIBILONI Y K. FREITAG.

La Plata, Bs As, Argentina

Workshop; 35th anniversary of Hyperfine Interactions at La Plata International Workshop; 2005

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Perturbed-Angular Correlations (PAC) and other hyperfine interaction measurements are widely used experimental techniques that provide information on the interaction of a probe-nucleus with the surrounding electronic charge distribution [1]. An interpretation of such measurements can lead to a detailed knowledge of structural, electronic, and magnetic properties of solids (see, e.g., Ref. 2). The measured quantities, the quadrupole coupling constant nQ and the asymmetry parameter h characterize the electric-field gradient tensor (EFG). Due to the r-3 dependence of the EFG, the EFG "felt" by a nucleus reflects sensitively the non-spherical electronic charge distribution around the nucleus. Therefore, the EFG is one of the most important clues for the understanding of the electronic structure in solids and the nature of the chemical bonding. Among the binary oxides, those that crystallize in the bixbyite structure were the subject of several PAC investigations using 111Cd and 181Ta as probe (see, e.g., Ref. 3 and references therein). In this work we report TDPAC experiments on 181Ta-implanted Eu2O3 and Ho2O3 polycristalline samples in order to complete the systematic study of the EFG at 181Ta atoms located at defect-free cation sites in the bixbyite structure. These results, as well as previous characterizations of the EFG at 181Ta sites in oxides with the bixbyite structure, were compared to those obtained in experiments using 111Cd as probe, to point-charge model (PCM) predictions and ab initio calculations results for the EFG tensor at impurity sites in binary oxides. These studies, in combination with the experimental ones, provide quantitative information about electronic processes and the structural relaxations induced by the presence of impurity probes in the host lattices, information that cannot be obtained (or is crudely estimated) by simple models such as the PCM, and confirm the existence of non-ionic contributions to the EFG in these systems.