Arvids Stashans

Quantum-chemical simulation of Al- and Sc-bound hole polarons in BaTiO3 crystal

Abstract Quantum-chemical modelling of impurity-bound hole polarons in BaTiO3 crystal was carried out employing the semi-empirical method of intermediate neglect of differential overlap (INDO) modified for crystals. Two-site hole has been found to be the most common configuration in the BaTiO3 crystal doped with Al or Sc atoms. The automated geometry optimisation was carried out to obtain the polaron spatial configuration and atomic displacements in the defective region. The ΔSCF calculated absorption energy for the aluminium-bound polaron was found to be equal to 0.27 eV and the corresponding absorption energies for scandium-bound polaron were computed to be 1.92 and 0.62 eV.

Defects in titanates

Electronic, structural and optical properties of titanates (BaTiO3, SrTiO3, CaTiO3 and PbTiO3) containing different point defects are reviewed and discussed in the present work. The results are obtained using a quantum-chemical method based on the Hartree-Fock formalism and utilising the Large Unit Cell (LUC) and the Embedded Molecular Cluster (EMC) models. The outcomes of the computations are compared and discussed in the light of the available experimental data. The formation of Jahn-Teller (JT) polarons and bipolarons due to the presence of some point defects in the titanate crystals are found to be very important in order to understand better the behaviour of these materials. In particular, such phenomena as the non-linear photoconductivity in BaTiO3, the drastic increase in the electrical conductivity in CaTiO3 and BaTiO3 crystals, and the superconductivity in SrTiO3 could be completely or partially explained by the presence of JT polarons and different interacting mechanisms between these defects.

Structural and electronic effects in BaTiO3 due to the Nb doping

Abstract Some effects in BaTiO3 due to the Nb-doping have been studied by means of the advanced quantum-chemical method based on the Hartree-Fock theory. A LUC (Larger Unit Cell) consisting of 135 atoms has been used. First, the effects in the cubic phase of the material have been investigated after the minimization of the total energy of the crystal. The obtained relaxation energy was found to be equal to 1.69 eV per LUC. An extra electron due to the Nb presence was found in the conduction band. In the tetragonal phase the relaxation energy is equal to 5.44 eV per LUC and an extra electron also jumps to the conduction band changing the state of the crystal.

DFT study of chromium-doped SnO 2 materials

Density functional theory (DFT) and generalised gradient approximation have been employed to study effects produced by chromium dopant in the tin dioxide. Hubbard-like term (DFT+U method) has been introduced to provide better description of magnetic moments, internal degrees of freedom and electronic band structure features. Results on microstructure, electronic properties and magnetic behaviour of the material are discussed for different dopant concentrations. A peak within the band-gap region has been found. It grows up in intensity for higher impurity concentrations. Hybridization between the Cr 3d and O 2p states within the upper valence band, detected in our study, might have an influence on magnetic behaviour of the Cr-doped SnO2 materials.

Oxygen-vacancy defects on BaTiO3 (001) surface: a quantum chemical study

Abstract A quantum-chemical study of technologically important BaTiO3 crystal and oxygen-vacancy defects on its (001) surface is reported in the present work. The computations are made using a quantum-chemical method developed for periodic systems (crystals), which is based on the Hartree–Fock theory. The atomic rearrangement due to the surface creation is obtained for a pure BaTiO3 by means of the periodic large unit cell (LUC) model and using an automated geometry optimisation procedure. The same technique is employed to study the electronic and structural properties of the material due to the presence of an O vacancy and F centre (two electrons trapped in an oxygen vacancy). The computations are carried out for both cubic and tetragonal lattices.

Electron transfer effect in BaTiO3 and CaTiO3 due to Nb-doping

Abstract Using an advanced quantum-chemical method, a study of Nb-doping has been carried out in the BaTiO 3 and CaTiO 3 crystals. The lattice distortion due to the impurity presence is studied in a comparative manner for the two crystals and also considering different crystallographic phases. A new phenomenon of an extra electron transfer from the local energy level within the band-gap to the conduction band is found. This effect is discussed in terms of the available experimental data on electrical conductivity augmentation in these materials.

Structural and electronic properties in cubic and tetragonal BaTiO3 crystal due to La impurity

Abstract The effects in BaTiO3 due to the La-doping have been studied by means of advanced quantum-chemical method based on the Hartree–Fock theory. A supercell consisting of 135 atoms has been used throughout the work. First, the effects in cubic phase of the material have been investigated after minimisation of the total energy of the crystal. The obtained relaxation energy was found to be equal to 0.94 eV. The extra electron due to the La atom was found to be situated in a local energy level within the band gap. In tetragonal phase the relaxation energy was obtained to be equal to 0.52 eV and again the extra electron is confined in the local energy level within the band gap.

Electronic structure, chemical bonding, and geometry of pure and Sr‐doped CaCO3

The electronic structure, chemical bonding, geometry, and effects produced by Sr‐doping in CaCO3 have been studied on the basis of density‐functional theory using the VASP simulation package and molecular‐orbital theory utilizing the CLUSTERD computer code. Two calcium carbonate structures which occur naturally in anhydrous crystalline forms, calcite and aragonite, were considered in the present investigation. The obtained diagrams of density of states show similar patterns for both materials. The spatial structures are computed and analyzed in comparison to the available experimental data. The electronic properties and atomic displacements because of the trace element Sr‐incorporation are discussed in a comparative manner for the two crystalline structures.

Hydrogen impurity in SrTiO3: structure, electronic properties and migration

The present paper reports a computational investigation of the geometry and electronic structure as well as the migration of a hydrogen impurity in the cubic SrTiO3 crystal. The study is done using an approach based on the Hartree–Fock theory and developed for periodic systems. It is found that the H impurity forms the so-called OH group at the equilibrium. Analysis of electron density within the defective region implies the enhancement in covalent chemical bonding. A possible defect migration has been also investigated.

LARGE HOLE POLARONS IN Sc-DOPED TiO2 CRYSTALS

First-principles calculations based on the density functional theory (DFT) within the generalized gradient approximation (GGA) have been used to study Sc-doped TiO2, rutile and anatase, crystals. Local defect microstructure, electronic and electrical properties have been obtained and discussed in the present work. Large radius hole polaron state found here points out to the possibility of p-type electrical conductivity in Sc-doped titania.