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

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

La Plata, Argentina

Workshop; HFILP2005, International Workshop “35th Anniversary of Hyperfine Interactions at La Plata”; 2005

Laboratorio de Espectroscopía Nuclear e Interacciones Hiperfinas, FCE, UNLP e IFLP (CONICET)

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 nuQ and the asymmetry parameter eta 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 theunderstanding 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 111 Cd and 181 Ta as probe (see, e.g., Ref. 3 and references therein). In this work we report TDPAC experiments on 181 Ta-implanted Eu2O3 and Ho2O3 polycristalline samples in order to complete the systematic study of the EFG at 181 Ta atoms located at defect-free cation sites in the bixbyite structure. These results, as well as previous characterizations of the EFG at 181 Ta sites in oxides with the bixbyite structure, were compared to those obtained in experiments using 111 Cd 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 thesesystems.[1] G. Schatz and A. Weidinger, in: Nuclear Condensed Matter Physics - NuclearMethods and Applications, ed. John Wiley & Sons, Chichester, England, 1996, p.63.[2] Proc. of the 12th International Conference on Hyperfine Interactions, Park City,Utah 2001, ed. by W.E. Evenson, H. Jaeger, and M.O. Zacate, Hyp. Interactions.[3] L.A. Errico, M. Rentería, A.F. Pasquevich, A.G. Bibiloni, and K. Freitag, Eur. Phys.J. B 22 , 149 (2001).Keywords : electric -field gradient, ionic model, ab initio, PAC spectroscopy, structuralrelaxations , oxide, FP-LAPW