Fe-doped SnO2: A Quantum-chemical Approach

We report first-principles results obtained on Fe impurity incorporation into the SnO2 material. Different impurity concentrations have been taken into consideration when computing structural, electronic and magnetic properties of the material. DFT + U methodology within the GGA approach applied to a 96-atom supercell allowed us to establish the equilibrium geometry of the system, which consists of six defectnearest oxygens shifting towards the Fe impurity. Antiparallel magnetic alignment between the electrons of the Fe 3d and impurity-neighbouring O 2p atomic orbitals forming the FeO6 complex has been found

Fe-doped SnO2: A Quantum-chemical Approach

We report first-principles results obtained on Fe impurity incorporation into the SnO2 material. Different impurity concentrations have been taken into consideration when computing structural, electronic and magnetic properties of the material. DFT + U methodology within the GGA approach applied to a 96-atom supercell allowed us to establish the equilibrium geometry of the system, which consists of six defectnearest oxygens shifting towards the Fe impurity. Antiparallel magnetic alignment between the electrons of the Fe 3d and impurity-neighbouring O 2p atomic orbitals forming the FeO6 complex has been found

Estudio químico-cuántico de los defectos producidos por las vacancias de oxígeno en los cristales de Titanato de plomo PbTiO3

Estudio químico-cuántico de los defectos producidos por las vacancias de oxígeno en los cristales de Titanato de plomo PbTiO

Li intercalation effects on magnetism in undoped and Co-doped anatase TiO2_2

The effects of $n$-type carrier doping by Li intercalation on magnetism in undoped and Co-doped anatase TiO$_2$ are investigated. We have found that doped $n$-type carriers in TiO$_2$ are localized mainly at Ti sites near the intercalated Li. With increasing the intercalation, local spins are realized at Ti. In the case of Co-doped TiO$_2$, most of the added $n$-type carriers fill the Co 3$d$ bands and the rest are localized at Ti. Therefore, Co magnetic moment vanishes by Li intercalation to have a nonmagnetic ground state.Comment: 6 pages, 2 figures, to appear in a special issue of Physica B, proceedings of the 2nd Hiroshima Workshop on Transport and Thermal Properties of Advanced Materials (Aug. 2002; Hiroshima, Japan

DFT Study of Intrinsic and Induced p-type Conductivity of ZnO Material

Density functional theory and generalized gradient approximation including a Hubbard-like term was used in the present work to analyse p-type electrical conductivity as well as the switch of n-type → p-type conductivity in the ZnO materials. Results on atomic shifts indicate significance of Coulomb electrostatic interaction in finding the equilibrium state of the system. It is shown that the p-type electrical conductivity could be obtained by the N impurity doping into the n-type ZnO samples and also by considering zinc vacancy defect in otherwise pure ZnO crystal. Computed concentrations of free-carriers for different samples are compared to the available experimental data

Theory of Coexistence of Superconductivity and Ferroelectricity : A Dynamical Symmetry Model

We propose and investigate a model for the coexistence of Superconductivity (SC) and Ferroelectricity (FE) based on the dynamical symmetries $su(2)$ for the pseudo-spin SC sector, $h(4)$ for the displaced oscillator FE sector, and $su(2) \otimes h(4)$ for the composite system. We assume a minimal symmetry-allowed coupling, and simplify the hamiltonian using a double mean field approximation (DMFA). A variational coherent state (VCS) trial wave-function is used for the ground state: the energy, and the relevant order parameters for SC and FE are obtained. For positive sign of the SC-FE coupling coefficient, a non-zero value of either order parameter can suppress the other (FE polarization suppresses SC and vice versa). This gives some support to "Matthias' Conjecture" [1964], that SC and FE tend to be mutually exclusive. For such a Ferroelectric Superconductor we predict: a) the SC gap $\Delta$ (and $T_c$) will increase with increasing applied pressure when pressure quenches FE as in many ferroelectrics, and b) the FE polarization will increase with increaesing magnetic field up to $H_c$. The last result is equivalent to the prediction of a new type of Magneto-Electric Effect in a coexistent SC-FE material. Some discussion will be given of the relation of these results to the cuprate superconductors.Comment: 46 page

Orthorhombic distortion on Li intercalation in anatase

Published versio

A roadmap of strain in doped anatase TiO2

Anatase titanium oxide is important for its high chemical stability and photocatalytic properties, however, the latter are plagued by its large band gap that limits its activity to only a small percentage of the solar spectrum. In that respect, straining the material can reduce its band gap increasing the photocatalytic activity of titanium oxide. We apply density functional theory with the introduction of the Hubbard + U model, to investigate the impact of stress on the electronic structure of anatase in conjunction with defect engineering by intrinsic defects (oxygen/titanium vacancies and interstitials), metallic dopants (iron, chromium) and non-metallic dopants (carbon, nitrogen). Here we show that both biaxial and uniaxial strain can reduce the band gap of undoped anatase with the use of biaxial strain being marginally more beneficial reducing the band gap up to 2.96 eV at a tensile stress of 8 GPa. Biaxial tensile stress in parallel with doping results in reduction of the band gap but also in the introduction of states deep inside the band gap mainly for interstitially doped anatase. Dopants in substitutional positions show reduced deep level traps. Chromium-doped anatase at a tensile stress of 8 GPa shows the most significant reduction of the band gap as the band gap reaches 2.4 eV

Comparative study of the implementation of tin and titanium oxide nanoparticles as electrodes materials in Li-ion batteries

Transition metal oxides potentially present higher specific capacities than the current anodes based on carbon, providing an increasing energy density as compared to commercial Li-ion batteries. However, many parameters could influence the performance of the batteries, which depend on the processing of the electrode materials leading to different surface properties, sizes or crystalline phases. In this work a comparative study of tin and titanium oxide nanoparticles synthesized by different methods, undoped or Li doped, used as single components or in mixed ratio, or alternatively forming a composite with graphene oxide have been tested demonstrating an enhancement in capacity with Li doping and better cyclability for mixed phases and composite anodes