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

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

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

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

Defects in TiO<inf>2</inf> crystals

TiO2 crystals, anatase and rutile, have been studied using Density Functional Theory (DFT) and the Generalized Gradient Approximation (GGA). Also, the intraatomic interaction term for strongly correlated d-electrons (DFT+U approximation) has been utilized for a proper description of the Ti d-electrons. The presence of some impurities in the otherwise pure crystal affects the structural, electronic and magnetic properties of the TiO2. The presence of a Fe atom in the anatase structure produces shifts of the Ti atoms towards it whereas the majority of O atoms move away. Also, it exhibits n-type electrical conductivity. When a Sc atom is present in the same structure, Ti atoms are not moving while the O atoms move outwards with respect to the impurity. There are no changes in the electrical conductivity. In the case of Fe atom incorporation into the rutile structure, the host Ti atoms move towards the point defect and O atoms move outwards, obviously due to the electrostatic forces. The bandgap reduction has been observed for this particular case. Finally, the presence of a Sc atom in the rutile crystalline lattice produces a local microstructure with all defectsurrounding atoms displacing themselves outwards the defect, most due to the reduction of the charge in the defective region. A small band-gap reduction (14%) has been also found in this case

Properties of the silver cyclic amide Ag2(C4H4NO2)2(H2O) crystal from the periodic DFT computations

© 2014 Pleiades Publishing, Ltd.A molecular crystal of silver cyclic amide Ag2(C4H4NO2)2(H2O) is studied using first-principles density functional theory calculations within the generalized gradient approximation (GGA). A number of different exchange-correlation functionals are considered for a possible treatment of the system. It is found that the Perdew-Burke-Ernzerhof (PBE) GGA exchange-correlation functionals are adequate for the Ag2(C4H4NO2)2(H2O) crystal. The results obtained show the possibility to reproduce rather well the geometry of at least some molecular crystals by means of the periodic solid-state calculations if the computational parameters are chosen adequately. The present work also reports the analysis of the chemical bonding in the material and gives the total and partial density of states. Our solid-state computations point out the possible magnetic properties of the molecular crystal under study