Ab Initio Study of Structural, Electronic, and Hyperfine Properties of n-type SnO2:Ta Semiconductor

A detailed theoretical first-principles study of structural, electronic, and hyperfine properties at Sn and Ta sites of undoped and Ta-doped rutile SnO2 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 181Ta-doped SnO2 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.Fil: Darriba, German Nicolas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; ArgentinaFil: Muñoz, Emiliano Luis. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; ArgentinaFil: Errico, Leonardo Antonio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; Argentina. Universidad Nacional del Noroeste de la Provincia de Buenos Aires; ArgentinaFil: Rentería, Mario. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; Argentin

Metal impurities in an oxide: Ab initio study of electronic and structural properties of Cd in rutile TiO₂

In this work we undertake the problem of a transition metal impurity in an oxide. We present an ab initio study of the relaxations introduced in TiO2 when a Cd impurity substitutionally replaces a Ti atom. Using the full-potential linearized-augmented-plane-wave method, we obtain relaxed structures for different charge states of the impurity and computed the electric-field gradients (EFGs) at the Cd site. We find that EFGs, and also relaxations, are dependent on the charge state of the impurity. This dependence is very remarkable in the case of the EFG and is explained by analyzing the electronic structure of the studied system. We predict fairly anisotropic relaxations for the nearest oxygen neighbors of the Cd impurity. The experimental confirmation of this prediction and a brief report of these calculations have recently been presented [Phys. Rev. Lett. 89, 55503 (2002)]. Our results for relaxations and EFGs are in clear contradiction with previous studies of this system that assumed isotropic relaxations, and point out that no simple model is viable to describe relaxations and the EFG at Cd in TiO2 even approximately.Departamento de Físic

FP-LAPW Study of the EFG at Impurity Sites in Oxides: Cd in Rutile TiO2

We report here first-principles determination of the electric-field gradient (EFG) tensor at the Cd impurity located at cation sites in rutile TiO2. As far as we know, these represent the first ab initio calculations at impurity sites with the FP-LAPW method in oxide systems. We used super-cells to simulate the diluted impurity in the crystal. The free-relaxation process performed in our study shows that the changes in distances of the oxygen nearest-neighbours to the impurity are not isotropic as was supposed in a previous study within the muffin-tin approximation. Our prediction for the EFG component of major absolute value agrees well with the experiment and is opposite in sign and direction if isotropic relaxations are assumed. Our value of the asymmetry parameter η also compares very well with the experimental value.Facultad de Ciencias Exacta

FP-LAPW Study of the EFG at Impurity Sites in Oxides: Cd in Rutile TiO2

We report here first-principles determination of the electric-field gradient (EFG) tensor at the Cd impurity located at cation sites in rutile TiO2. As far as we know, these represent the first ab initio calculations at impurity sites with the FP-LAPW method in oxide systems. We used super-cells to simulate the diluted impurity in the crystal. The free-relaxation process performed in our study shows that the changes in distances of the oxygen nearest-neighbours to the impurity are not isotropic as was supposed in a previous study within the muffin-tin approximation. Our prediction for the EFG component of major absolute value agrees well with the experiment and is opposite in sign and direction if isotropic relaxations are assumed. Our value of the asymmetry parameter η also compares very well with the experimental value.Facultad de Ciencias Exacta

Ab initio LSDA and LSDA+U study of pure and Cd-doped cubic lanthanide sesquioxides

The electronic, structural, and hyperfine properties of pure and Cd-doped lanthanide (Ln) sesquioxides with the cubic bixbyite structure (Ln2O3, 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 In2O3, Sc2O3, and Lu2O3 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.Fil: Richard, Diego. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; ArgentinaFil: Muñoz, Emiliano Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; ArgentinaFil: Rentería, Mario. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; ArgentinaFil: Errico, Leonardo Antonio. Universidad Nacional del Noroeste de la Provincia de Buenos Aires; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Departamento de Física; ArgentinaFil: Svane, A.. University Aarhus. Institut for Fysik Og Astronomi; DinamarcaFil: Christensen, N. E.. University Aarhus. Institut for Fysik Og Astronomi; Dinamarc

The role of vacancies, impurities and crystal structure in the magnetic properties of TiO₂

We present an ab initio study of pure and doped TiO₂ in the rutile and anatase phases. The main purpose of this work is to determine the role played by different defects and different crystal structures in the appearance of magnetic order. The calculations were performed for varying impurity and vacancy concentrations in both TiO₂ structures. For Co impurities the local magnetic moment remained almost independent of the concentration and distribution while for Cu this is not the case, there is magnetism for low concentrations that disappears for the higher ones. Impurity–impurity interactions in both structures favor linear ordering of them. Magnetism in undoped samples appears for certain vacancy concentrations and structural strain.Facultad de Ciencias ExactasInstituto de Física La Plat

Ab Initio Study of Structural, Electronic, and Hyperfine Properties of n-type SnO2:Ta Semiconductor

A detailed theoretical first-principles study of structural, electronic, and hyperfine properties at Sn and Ta sites of undoped and Ta-doped rutile SnO2 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 181Ta-doped SnO2 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.Instituto de Física La Plat

FP-LAPW Study of the EFG at Impurity Sites in Oxides: Cd in Rutile TiO2

We report here first-principles determination of the electric-field gradient (EFG) tensor at the Cd impurity located at cation sites in rutile TiO2. As far as we know, these represent the first ab initio calculations at impurity sites with the FP-LAPW method in oxide systems. We used super-cells to simulate the diluted impurity in the crystal. The free-relaxation process performed in our study shows that the changes in distances of the oxygen nearest-neighbours to the impurity are not isotropic as was supposed in a previous study within the muffin-tin approximation. Our prediction for the EFG component of major absolute value agrees well with the experiment and is opposite in sign and direction if isotropic relaxations are assumed. Our value of the asymmetry parameter η also compares very well with the experimental value.Facultad de Ciencias Exacta

The role of metallic impurities in oxide semiconductors: first‐principles calculations and PAC experiments

We report an ab-initio comparative study of the electric-field-gradient tensor (EFG) and structural relaxations introduced by acceptor (Cd) and donor (Ta) impurities when they replace cations in a series of binary oxides: TiO₂, SnO₂, and In₂O₃. Calculations were performed with the Full-Potential Linearized-Augmented Plane Waves method that allows us to treat the electronic structure and the atomic relaxations in a fully self-consistent way. We considered different charge states for each impurity and studied the dependence on these charge states of the electronic properties and the structural relaxations. Our results are compared with available data coming from PAC experiments and previous calculations, allowing us to obtain a new insight on the role that metal impurities play in oxide semiconductors. It is clear from our results that simple models can not describe the measured EFGs at impurities in oxides even approximately.Facultad de Ciencias Exacta

Intercambio por reacción en fase sólida de impurezas Hf en sitios de catión del semiconductor de ancho Gap C-Tm₂O₃

En este trabajo estudiamos el intercambio de impurezas donoras Hf en sitios sustitucionales de catión del semiconductor de ancho gap C-Tm₂O₃. El proceso de dopaje se realizó por reacción en fase sólida, asistida por molido mecánico, de m-HfO₂ (activado por irradiación neutrónica) y de C-Tm₂O₃. Atomos 181Ta, obtenidos en el decaimiento (β‘ del 181Hf (indistinguible de los átomos de Hf inactivos desde el punto de vista del intercambio iónico), fueron utilizados como sondas en los experimentos de Correlaciones Angulares Perturbadas Diferenciales en Tiempo realizados luego de cada etapa del proceso de dopaje. Las interacciones hiperfmas medidas en sitios 181Ta permitieron la caracterización del gradiente de campo eléctrico (GCE) en sitios representativos de la localización del Hf en cada etapa del proceso. La eficiencia y el carácter sustitucional del proceso de intercambio es discutido y demostrado a la luz de una sistemática establecida para GCEs en óxidos de tierras raras isoestructurales (bixbitas).The ionic exchange of donor Hf impurities at substitutional cationic sites ofthe cubic (bixbyite) phase of the wide- gap semiconductor C-Tm₂O₃ was studied. The doping process was performed by ball-milling-assisted solid-state reaction of neutron-activated m-HfO₂ and C-Tm₂O₃. 181Ta atoms, obtained by β‘-decay of the 181Hf-isotope, were used as probes in Time-Differential Perturbed-Angular-Correlation experiments carried out alter each step ofthe doping process. The measured hyperfme interactions at 181Ta sites enabled the electric-field gradient (EFG) characterization at representativo Hf sites of each step of the process. The efficiency and substitutional character of the exchange process is discussed and elucidated in the frame of an empirical EFG systematics established in isostructural rare-earth oxides (bixbyites).Facultad de Ciencias Exacta