A detailed theoretical first-principles study of structural, electronic, and hyperfine properties at Sn and Ta sites of undoped and Ta-doped rutile SnO 2 is presented, using the Full-Potential Augmented Plane Wave plus local orbitals (FP-APW+lo) method. In the Ta-doped systems, we performed calculations for two different charge states. The predicted electric-field-gradient (EFG) tensor, the key magnitude in this study, for both charge states of the impurity result to be almost equal and in good agreement with Time-Differential Perturbed γ−γ Angular Correlation (TDPAC) results in 181 Ta-doped SnO 2 thin films. This study enables at present to discuss the origin of the EFG and the role played by the structural anisotropic contractions introduced by the Ta atom and the impurity charge state on the hyperfine properties. To determine the correct charge state of the impurity, we performed energetic studies, predicting the metallic behavior of degenerate semiconductors, in agreement with resistivity experimental results obtained in samples with the same Ta dilution.

The electronic, structural, and hyperfine properties of pure and Cd-doped lanthanide (Ln) sesquioxides with the cubic bixbyite structure (Ln 2 O 3 , Ln ranging from La to Lu) have been studied using the full-potential augmented plane wave plus local orbital (APW + lo) method within the local spin density approximation (LSDA) and the Coulomb-corrected LSDA + U . In the case of the pure systems, our calculations show that LSDA + U gives a better representation of the band structure compared to LSDA. The predicted equilibrium structures and the electric field gradient (EFG) tensor at Ln sites were calculated and compared with those obtained by means of hyperfine techniques and with theoretical results obtained in In 2 O 3 , Sc 2 O 3 , and Lu 2 O 3 reported in the literature. The origin of the EFG at Ln sites and the role played by the 4f electrons on this quantity are discussed. In the case of the Cd-doped systems, the APW + lo method (also within LSDA and LSDA + U ) was applied to treat the electronic structure of the doped system. The role of the Ln 4f electrons on the EFG at Cd impurity sites, and other variables like structural distortions induced by the Cd impurity, were investigated in detail and are discussed and compared with available experimental results. An excellent agreement between the experimental and calculated EFGs was found for all Cd-doped systems.

In this work we present an experimental and theoretical study from first-principles of the structural, electronic, and hyperfine properties of Ta-doped In2O3 semiconductor. The ab initio electronic structure calculations in the Ta-diluted In2O3 system enabled to obtain the structural lattice distortions and the hyperfine parameters when the Ta atom is placed at each cationic site of the bixbyite crystal structure. To this purpose we used the full-potential augmented plane wave plus local orbital (FP-APW+lo) method, within the density functional theory. The obtained results indicate that the substitutional Ta probe-impurity produces strong changes on the local structure. In addition, we performed accurate time-differential perturbed γ–γ angular correlations (TDPAC) key experiments in 181Hf­(→181Ta)-implanted In2O3 samples with high crystallinity, in order to obtain high quality measurements of the electric-field gradient tensor (EFG) that unraveled the controversy settled in the literature and overcome dissimilar interpretations of previously reported TDPAC experiments. The experiments were performed at room temperature in air, after each step of a series of thermal annealing treatments in air at increasing temperatures in order to remove radiation damage and locate the 181Hf probes at substitutional cationic sites. We succeeded to obtained two well-defined hyperfine interactions that were assigned to 181Ta probes located at the two defect-free inequivalent cationic sites of the In2O3 crystal structure. The EFG calculations are in excellent agreement with the results of these TDPAC measurements, and show that the largest component of the diagonalized EFG, V 33, at the Ta site has mainly p character. The accuracy of the experiments together with the reliable and precise ab initio results allowed a definitive determination of the EFG at both cationic sites in this system. Formation energy calculations of defects were needed to determine the charge state of the 181Ta impurity, which agrees with a semiconducting character for the In2O3:Ta doped system.

The time-differential ␥-␥ perturbed-angular-correlation ͑TDPAC͒ technique using 44 Ti → 44 Sc tracers was applied to study the nuclear quadrupole interaction of the first excited I = 1 state of 44 Sc in the cubic bixbyite structure of scandium sesquioxide ͑Sc 2 O 3 ͒. In addition, ab initio calculations of electronic and structural properties and hyperfine parameters at the cationic sites of the Sc 2 O 3 structure were performed using the full-potential augmented plane wave plus local-orbital ͑APW+ lo͒ method. The accuracy of the calculations and the excellent agreement of the predicted electric-field-gradient ͑EFG͒ tensors and the structural properties ͑lattice parameters, internal positions͒ with the experimental results enable us to identify the observed hyperfine interactions and to infer the EFG sign that cannot be measured in conventional TDPAC experiments. Additionally, the APW+ lo calculations show that the EFG at Sc sites is originated in the population of Sc 3p states and give an explanation for the preferential occupation of the asymmetric cationic site C of the structure by the 44 Ti doping impurities. Finally, the validity of the ionic model, usually used to describe the EFG at native cation sites, is discussed.

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We report Perturbed-Angular-Correlation (PAC) experiments on corundum Al 2 O 3 single crystals implanted with 181 Hf/ 181 Ta ions at the ISKP at Bonn and measured at La Plata with high efficiency and timeresolution. The magnitude, asymmetry, and orientation (with respect to the crystalline axes) of the electric-field gradient (EFG) tensor were determined measuring the spin-rotation curves as a function of different orientations of the single crystals relative to the detector system. These results are analyzed in the framework of point-charge model and ab initio Full-Potential Linearized-Augmented Plane Wave calculations, and compared with EFG results coming from PAC experiments with 111 In/ 111Cd impurities. This combined study enables the determination of lattice relaxations induced by the presence of the impurity and the state of charge of a deep impurity donor level in the band gap of the semiconductor.

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The combination of hyperfine techniques and ab initio calculations has been shown to be a powerful tool to unravel structural and electronic characterizations of impurities in solids. A recent example has been the study of Cddoped SnO, where ab initio calculations questioned previous TDPAC assignments of the electric-field gradient (EFG) in 111 In-implanted Sn-O thin films. Here we present new TDPAC experiments at 111 In-difused polycrystalline SnO. A reversible temperature dependence of the EFG was observed in the range 295-900 K. The TDPAC results were compared with theoretical calculations performed with the fullpotential linearized augmented plane wave (FP-LAPW) method, in the framework of the density functional theory. Through the comparison with the theoretical results, E. L. Muñoz (B) · L. A. Errico · M. Rentería E.L. Muñoz et al.we infer that different electronic surroundings around Cd impurities can coexist in the SnO sample.