The, time-differential $\gamma \text{−}\gamma$ perturbed-angular-correlation (TDPAC) technique using ion-implanted ${}^{181}\text{H}\text{f}\left(\to {}^{181}\text{T}\text{a}\right)$ tracers was applied to study the hyperfine interactions of ${}^{181}\text{T}\text{a}$ impurities in the rutile structure of ${\text{TiO}}_2$ single crystals. The experiments were performed in air in the temperature range of 300–1273 K, allowing the electric-field-gradient (EFG) tensor characterization (in magnitude, asymmetry, and orientation) at ${}^{181}\text{T}\text{a}$ probe atoms located in defect-free cation sites of the structure. The measured EFG is parallel to the [001] crystal axis, as occurs at Ti sites, but normal to the EFG orientation observed at ${}^{111}\text{C}\text{d}$ impurities in ${\text{TiO}}_2$ single crystals [L. A. Errico et al., Phys. Rev. Lett. 89, 055503 (2002)]. In addition, ab initio calculations were performed using the full-potential augmented plane wave plus local orbital method that allow us to treat the electronic structure of the doped system and the atomic relaxations induced by the Ta impurity in a fully self-consistent way. We considered different dilutions of the doped system (using the supercell approach) and studied the electronic properties and structural atomic relaxation dependence on the charge state of the impurity. The accuracy of the calculations and the excellent agreement of the predicted magnitude, asymmetry, and orientation of the EFG tensor with the experimental results enable us to infer the EFG sign, not accessible with conventional TDPAC experiments. The comparison of the measured EFG at Ta sites with experimental and ab initio theoretical results reported in the literature at Cd, Ta, and Ti sites in ${\text{TiO}}_2$ allowed us to obtain a deeper insight on the role played by metal impurities in oxide semiconductors.

• Received 30 December 2008

DOI:https://doi.org/10.1103/PhysRevB.79.115213