Titania nanotubes modeled from 3- and 6-layered (101) anatase sheets: Line group symmetry and comparative ab initio LCAO calculations

Abstract The formalism of line groups for one-periodic (1D) nanostructures with rotohelical symmetry has been applied for construction of TiO 2 nanotubes (NTs). They are formed by rolling up the stoichiometric two-periodic (2D) sheets cut from the energetically stable (1 0 1) anatase surface, which contains either six (O–Ti–O_O–Ti–O) or three (O–Ti–O) layers. After optimization of geometry the former keeps the centered rectangular symmetry of initial slab while the latter is spontaneously reconstructed to the hexagonal fluorite-type (1 1 1) sheet. We have considered the four sets of TiO 2 NTs with optimized 6- and 3-layered structures, which possess the two pairs of either anatase (− n , n …

Ab initiosimulations on rutile-based titania nanowires

The rod symmetry groups for monoperiodic (1D) nanostructures have been applied for construction of models for bulk-like TiO2 nanowires (NWs) cut from a rutile-based 3D crystal along the chosen [001] and [110] directions of crystallographic axes. In this study, we have considered nanowires described by both the Ti-atom centered rotation axes as well as the hollow site centered axes passing through the interstitial positions between the Ti and O atoms closest to the axes. The most stable [001]-oriented TiO2 NWs with rhombic cross sections are found to display the energetically preferable {110} facets only while the nanowires with quasi-square sections across the [110] axis are formed by the a…

Use of site symmetry in supercell models of defective crystals: Polarons in CeO2

The authors thank R. Merkle and G. W. Watson for stimulating discussions. E. K. also acknowledges partial financial support from the Russian Science Foundation for the study of charged defects under the project 14-43-00052. A. C. also acknowledges financial support from the University of Latvia Foundation (Arnis Riekstins's "MikroTik" donation). E. K. and D. G. express their gratitude to the High Performance Computer Centre in Stuttgart (HLRS, project DEFTD 12939) for the provided computer facilities whereas R. A. E. thanks the St. Petersburg State University Computer Center for assistance in high-performance calculations.

First-principles comparative study of perfect and defective CsPbX3 (X = Br, I) crystals

We thank R. Merkle for numerous fruitful discussions and G. Siegle for experimental assistance. This study was partly supported by the M-ERA-NET project SunToChem (EK). Calculations were performed using computational facilities of St. Petersburg State University and Max Planck Institute for Solid State Research. Open Access funding provided by the Max Planck Society.

First principles evaluation on photocatalytic suitability of 2H structured and [0001] oriented WS2 nanosheets and nanotubes

This study was supported by the EC ERA.Net RUS Plus Project No. 237 WATERSPLIT. R.E. acknowledges the financial support provided by the Russian Foundation for Basic Research (grant N 17-03-00130a) and High Performance Computer Center of St. Petersburg University for the assistance. The authors are indebted to D. Bocharov, O. Lisovski and E. Spohr for stimulating discussions.

First-Principles Evaluation of the Morphology of WS2 Nanotubes for Application as Visible-Light-Driven Water-Splitting Photocatalysts

This study was supported by the EC ERA.Net RUS Plus project No. 237 WATERSPLIT as well as Russian Basic Research Foundation No. 16-53-76019. S.K. and E.S. furthermore gratefully acknowledge computing time granted by the Center for Computational Sciences and Simulation (CCSS) of the Universitaẗ Duisburg-Essen and the supercomputer magnitUDE (DFG grants INST 20876/209-1 FUGG, INST 20876/243-1 FUGG) provided by the Zentrum für Informations-und Mediendienste (ZIM). E.S. is also grateful for support by the Cluster of Excellence RESOLV (EXC1069) funded by the Deutsche Forschungsgemeinschaft.

Ab initio Hartree-Fock calculations of LaMnO3 (110) surfaces

We present the results of ab initio Hartree-Fock calculations of the LaMnO3 (110) surface. Using seven-plane slabs, periodic in 2D and containing three formula units, we compare the properties of a stoichiometric surface with oxygen vacancies, and non-stoichiometric, defect-free surfaces, analyze the dispersion of the effective charges near the surface, and calculate the surface energy before and after relaxation, for both ferromagnetric and antiferromagnetic spin orderings in a slab. q 2003 Elsevier Ltd. All rights reserved.

Site symmetry approach applied to the supercell model of MgAl2O4 spinel with oxygen interstitials: Ab initio calculations

This study has been carried out within the framework of the EUROfusion Consortium and has been provided funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053. The authors are indebted to E.A. Kotomin, A.I. Popov and R. Vila for stimulating discussions. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Calculations have been performed using both the Marconi supercomputer system at the Computational Simulation Centre (Italy) and the Computer Center of St. Petersburg State University.

Four-faceted nanowires generated from densely-packed TiO2 rutile surfaces: Ab initio calculations

Abstract Two-dimensional (2D) slabs and monoperiodic (1D) nanowires orthogonal to the slab surface of rutile-based TiO 2 structure terminated by densely-packed surfaces and facets, respectively, have been simulated in the current study. The procedure of structural generation of nanowires (NWs) from titania slabs (2D → 1D) is described. We have simulated: ( i ) (110), (100), (101) and (001) slabs of different thicknesses as well as ( ii ) [001]- and [110]-oriented nanowires of different diameters terminated by either four types of related {110} facets or alternating { 1 1 ¯ 0 } and {001} facets, respectively. Nanowires have been described using both the Ti atom-centered rotation axes as well…

First-principles calculations of iodine-related point defects in CsPbI3

Many thanks to A. Lushchik, A. Popov and R. Merkle for numerous fruitful discussions. This study was partly supported by the Latvian Council for Science (grant LZP-2018/1-0147 to EK). R.A.E acknowledges the assistance of the University Computer Center of Saint-Petersburg State University for high-performance computations.

First-principles calculations of the atomic and electronic structure ofFcenters in the bulk and on the (001) surface ofSrTiO3

The atomic and electronic structure, formation energy, and the energy barriers for migration have been calculated for the neutral O vacancy point defect F center in cubic SrTiO3 employing various implementations of density functional theory DFT. Both bulk and TiO2-terminated 001 surface F centers have been considered. Supercells of different shapes containing up to 320 atoms have been employed. The limit of an isolated single oxygen vacancy in the bulk corresponds to a 270-atom supercell, in contrast to commonly used supercells containing 40– 80 atoms. Calculations carried out with the hybrid B3PW functional show that the F center level approaches the conduction band bottom to within 0.5 eV…

Confinement effects for ionic carriers in SrTiO3 ultrathin films: first-principles calculations of oxygen vacancies.

One-dimensional confinement effects are modelled within the hybrid HF-DFT LCAO approach considering neutral and single-charged oxygen vacancies in SrTiO(3) ultrathin films. The calculations reveal that confinement effects are surprisingly short-range in this partly covalent perovskite; already for film thickness of 2-3 nm (and we believe, similar size nanoparticles) only the surface-plane defect properties differ from those in the bulk. This includes a pronounced decrease of the defect formation energy (by ∼1 eV), a much deeper defect band level and a noticeable change in the electronic density redistribution at the near-surface vacancy site with respect to that in the bulk. The results als…

Ab initio LCAO study of the atomic, electronic and magnetic structures and the lattice dynamics of triclinic CuWO4

Abstract The electronic, structural and phonon properties of antiferromagnetic triclinic CuWO 4 have been studied using the first-principles spin-polarized linear combination of atomic orbital (LCAO) calculations based on the hybrid exchange–correlation density functional (DFT)/Hartree–Fock (HF) scheme. In addition, the local atomic structure around both Cu and W atoms has been probed using extended X-ray absorption fine structure (EXAFS) spectroscopy. We show that, by using the hybrid DFT–HF functional, one can accurately and simultaneously describe the atomic structure (the unit cell parameters and the atomic fractional coordinates), the band gap and the phonon frequencies. In agreement w…

Ab initio calculations of charged point defects in GaN

The Generalized Gradient Approximation (GGA) of a Density Functional Theory (DFT) in LCAO basis is used to determine electronic structure of charged point defects in hexagonal GaN. Impurities of MgGa, ZnGa, SiN and CN (the most common dopants) were considered for charge states 0, ±1, –2. The vacancies of VGa and VN were considered for charge states 0 and 0, ±1, ±2 respectively. The defects in consideration are compared from the point of view of the one-electron states localization. All the calculations were performed for a supercell containing 96 atoms. The divergence due to periodically repeated charges has been eliminated by the use of a uniform background charge of an opposite sign. (© 2…

DFT study of a singleF center in cubic SrTiO3 perovskite

Various properties of a cubic phase of SrTiO3 perovskite containing single F centers (neutral oxygen vacancies), including energies of their formation and migration, were simulated using different formalisms of density functional theory (DFT) as implemented into CRYSTAL-2003 and VASP computer codes. The lattice relaxation around the F center was found to be sensitive to both shape and size of supercells used. The larger the supercell, the closer the defect energy level in the bandgap lies to the conduction band bottom. It approaches the optical ionization energy of 0.49 eV for 270- and 320-atom supercells, where the distance between neighboring defects increases up to four lattice constants…

First-principles calculations on Fe-Pt nanoclusters of various morphologies

Financial support provided by Scientific Research Project for Students and Young Researchers Nr. SJZ/2016/17 implemented at the Institute of Solid State Physics, University of Latvia, is greatly acknowledged. A.P. and R.E. express their gratitude to High-performance computer centers of ISSP (University of Latvia) and St. Petersburg University. This research was partially supported by Graphene Flagship GrapheneCore1-AMD-696656-4.

Quantum chemical simulations of doped ZnO nanowires for photocatalytic hydrogen generation

Zinc oxide (ZnO) is considered in general as a promising material for solar water splitting. Its wurtzite-structured bulk samples, however, can be considered as active for photocatalytic applications only under UV irradiation, where they possess ∼1% efficiency of sunlight energy conversion due to their wide band gap (3.4 eV). Although pristine ZnO nanowires (NWs) possess noticeably narrower band gaps than the bulk, the tendency of band gap reduction with increasing NW diameter is insufficient, and further modification is required. We have contributed to filling this gap by performing a series of ab initio calculations on ZnO NWs of different diameters (dNW), which are mono-doped by metal (A…

Topological analysis of chemical bonding in the layered FePSe3 upon pressure-induced phase transitions

The authors acknowledge the assistance of the University Computer Center of Saint-Petersburg State University in the accomplishment of high-performance computations. A.K. is grateful to the Latvian Council of Science project no. lzp-2018/2-0353 for financial support. Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2.

First-principles study of bulk and surface oxygen vacancies in SrTiO3 crystal

The structural and electronic properties of the neutral and positively charged oxygen vacancies (F and F + centres) in the bulk and on the (001) surfaces of SrTiO3 crystal are examined within the hybrid Hartree-Fock and density functional theory (HF-DFT) method based upon the linear combination of atomic orbital (LCAO) approach. A comparison of the formation energy for surface and bulk defects indicates a perceptible propensity for the segregation of neutral and charged vacancies to both SrO and TiO2 surface terminations with a preference in the latter case which is important for interpretation of space charge effects at ceramic interfaces. It is found that the vacancies reveal more shallow…

Periodic models in quantum chemical simulations ofF centers in crystalline metal oxides

We present a survey of recent first principles simulations of the neutral oxygen vacancies (F centers) existing as native or radiation-induced point defects in various crystalline metal oxides in different forms (bulk, bare substrate surface, and on the interface with metal adsorbates). We mainly consider periodic models in calculations of point defects using the metal oxide supercell or cyclic clusters. We compare different formalisms of first principles calculations, mostly the Density Functional Theory (DFT) as implemented in the framework of either localized basis set of atomic orbitals or delocalized basis sets of plane waves. We analyze in detail the structural and electronic properti…

Ab initio study of the electronic and atomic structure of the wolframite-type ZnWO4

Abstract Ab initio quantum chemistry calculations of the structural and electronic properties of monoclinic wolframite-type ZnWO 4 crystal have been performed within the periodic linear combination of atomic orbitals (LCAO) method using six different Hamiltonians, based on density functional theory (DFT) and hybrid Hartree-Fock-DFT theory. The obtained results for optimized structural parameters, band gap and partial density of states are compared with available experimental data, and the best agreement is observed for hybrid Hamiltonians. The calculations show that zinc tungstate is a wide band gap material, with the direct gap about 4.6 eV, whose valence band has largely O 2p character, w…

Origin of pressure-induced insulator-to-metal transition in the van der Waals compound FePS3 from first-principles calculations

The authors acknowledge the assistance of the University Computer Center of Saint‐Petersburg State University in the accomplishment of high‐performance computations. A.K. is grateful to the Latvian Council of Science project no. lzp‐2018/2‐0353 for financial support.

Symmetry and Models of Double-Wall BN and TiO2 Nanotubes with Hexagonal Morphology

The line symmetry groups for one-periodic (1D) nanostructures with rotohelical symmetry have been applied for symmetry analysis of double-wall boron nitride and titania nano- tubes (DW BN and TiO2 NTs) formed by rolling up the stoichiometric two-periodic (2D) slabs of hexagonal structure with the same or opposite orientation of translation and chiral vectors. We have considered the two sets of commensurate DW BN and TiO2 NTs with either armchair- or zigzag-type chiralities, i.e., (n1,n1)@(n2,n2 )o r (n1,0)@(n2,0), respectively. To establish theequilibriuminterwalldistancescorrespondingtotheminimaof energy, we have varied chiral indices n1 and n2 of the constituent single-wall (SW) nanotubes…

Symmetry and Stability of the Rutile-Based TiO2 Nanowires: Models and Comparative LCAO-Plane Wave DFT Calculations

The rod symmetry groups for monoperiodic (1D) nanostructures have been applied for construction of models for bulk-like titania nanowires (NWs) cut from a rutile-based 3D crystal along the direction of a chosen crystallographic symmetry axis (in this study we consider only Ti atom-centered axes). The most stable [001]-oriented TiO2 NWs with rhombic cross sections are found to display the energetically preferable {110} facets only, while the nanowires with quasi-square sections across the [110] axis are formed by the alternating {110} and {001} facets. For simulations on rutile-based nanowires possessing four different diameters for each NW type, we have performed comparative large-scale ab …

A Comparative Ab Initio Thermodynamic Study of Oxygen Vacancies in ZnO and SrTiO3: Emphasis on Phonon Contribution

Using a hybrid Hartree–Fock (HF)-DFT method combined with LCAO basis set and periodic supercell approach, the atomic, electronic structure and phonon properties of oxygen vacancies in ZnO and SrTiO3 were calculated and compared. The important role of a ghost basis function centered at the vacant site and defect spin state for SrTiO3 is discussed. It is shown that the use of hybrid functionals is vital for correct reproduction of defects basic properties. The Gibbs free energy of formation of oxygen vacancies and their considerable temperature dependence has been compared for the two oxides. These calculations were based on the polarizability model for the soft mode temperature behavior in S…

First-principles calculations of oxygen interstitials in corundum: a site symmetry approach

The authors are indebted to R. Vila, A. Popov and A. Lushchik for stimulating discussions. This work was carried out within the framework of the EUROfusion Consortium and received funding from the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Support from Latvian National Research Program IMIS2 (2014–2017) is also appreciated. Calculations were carried out using both the Marconi supercomputer system at the Computational Simulation Centre and the Computer Center of St. Petersburg State University.

A first-principles DFT study of UN bulk and (001) surface: comparative LCAO and PW calculations.

LCAO and PW DFT calculations of the lattice constant, bulk modulus, cohesive energy, charge distribu- tion, band structure, and DOS for UN single crystal are analyzed. It is demonstrated that a choice of the uranium atom relativistic effective core potentials considerably affects the band structure and magnetic structure at low tem- peratures. All calculations indicate mixed metallic-covalent chemical bonding in UN crystal with U5f states near the Fermi level. On the basis of the experience accumulated in UN bulk simulations, we compare the atomic and elec- tronic structure as well as the formation energy for UN(001) surface calculated on slabs of different thickness using both DFT approach…

FIRST-PRINCIPLES LCAO CALCULATIONS ON 5D TRANSITION METAL OXIDES: ELECTRONIC AND PHONON PROPERTIES

ABSTRACT First-principles quantum chemistry calculations within the periodic linear combination of atomic orbitals (LCAO) formalism have been used to probe electronic and phonon properties of crystalline 5d transition metal oxides ReO3, ZnWO4 and CaWO4. The obtained equilibrium crystal structure is in good agreement with known crystallographic data. Rhenium trioxide is correctly predicted to be a metal, whereas both tungstates—a wide gap insulating compounds. The phonon frequencies, calculated by the frozen phonon method, agree rather well with those obtained by infrared and Raman spectroscopies.

Quantum mechanics-molecular dynamics approach to the interpretation of x-ray absorption spectra.

The quantum mechanics-molecular dynamics approach to the simulation of configuration-averaged EXAFS spectra is proposed, and its application is discussed for the example of the Ti K-edge EXAFS spectrum in cubic perovskite SrTiO(3). Proper use of ab initio quantum mechanics allows a number of empirical parameters, used in the molecular dynamics simulation, to be reduced, whereas the molecular dynamics allows us to account for temperature effects. All together, the approach provides a way of accounting for static and dynamic disorder in EXAFS signals from the coordination shells above the first one, where many-atom (multiple-scattering) effects are often important.

Jahn-Teller effect in the phonon properties of defective SrTiO3from first principles

the Jahn–Teller effect occurs, thus reducing the cubic symmetry of a perfect crystal and leading to the appearance of both Raman- and infrared-active vibrational modes. The calculated phonon densities of states and group-theoretical analysis of defect-induced phonon frequencies were used for the interpretation of the relevant experimental data, once defect-induced local modes are identified. The temperature dependence of the Vo formation energy based on the calculated Gibbs free energy was also compared with experiments, and the phonon contribution therein estimated.

Quantum mechanics-classical molecular dynamics approach to EXAFS

Recently developed approach to the simulation of configuration-averaged EXAFS spectra using the combination of quantum mechanics and classical Molecular Dynamics (MD) methods is presented on the example of the Ti K-edge in SrTiO3 at T = 300 K. The method allows one to significantly reduce the number of fitting parameters required in the EXAFS signal calculation and to account entirely for disorder contributions. We show also that the sensitivity of configuration-averaged EXAFS spectra to the force field model employed in the MD simulations allows one to use them as additional information for the force field parameters fitting.

<title>Large-scale first-principles calculations of Fe-doped SrTiO<formula><inf><roman>3</roman></inf></formula></title>

The energy level positions in the optical gap and atomic geometry for the Fe4+ impurity substituting for a host Ti atom in SrTiO3 are calculated using the Unrestricted Hartree-Fock (UHF) method and supercells containing up to 320 atoms. In agreement with experiment, the high spin (S = 2) state is much lower in energy than the zero-spin state. The energy level positions strongly depend on the asymmetric displacements of six nearest O ions which is a combination of the Jahn-Teller and breathing modes. A considerable covalent bonding between the Fe ion and four nearest O ions takes place. We predict a strong dependence of optical absorption energies on the crystal compression or internal tensi…

LCAO calculation of neutral defects in GaN

Four well known HF, LDA, GGA and B3LYP Hamiltonians in LCAO approximation have been used in band structure calculations to obtain the main properties of the perfect GaN crystal with hexagonal lattice (C space group). Calculated lattice parameters, elastic constants and the band gap have been compared with the experimental data and the results of other calculations. As a consequence, the GGA Hamiltonian has been chosen, giving the lattice parameters a = 3.20 A, c = 5.20 A, u = 0.377, the bulk modulus B = 206 GPa and the energy gap Eg = 2.7 eV. These results reasonably reproduce the experimental data. For the point defects calculation (VGa, VN, MgGa, ZnGa, CN, and SiN) the supercell model was…

Site symmetry approach in the supercell model of carbon-doped ZnO bulk

Abstract Carbon-doped zinc oxide is one of promising materials for technological applications due to its ferromagnetism observed at room temperature. When using the hybrid DFT-HF Hamiltonian based on the PBE0 exchange-correlation functional for large-scale calculations on defective ZnO:C single crystal, we have shown that application of supercell model for carbon impurity located at O site of wurtzite-structured ZnO bulk results in the dependence of calculated formation energy of the point defect (Eform) on the selected site symmetry of the substituted atom in the supercell. For a more symmetric C3v site usually used for simulation of defective ZnO structures, values of formation energy per…

Interpretation of unexpected behavior of infrared absorption spectra ofScF3beyond the quasiharmonic approximation

Scandium fluoride (${\mathrm{ScF}}_{3}$), having cubic ${\mathrm{ReO}}_{3}$-type structure, has attracted much scientific attention due to its rather strong negative thermal expansion (NTE) in the broad temperature range from 10 to 1100 K. Here we use the results of diffraction and extended x-ray absorption fine-structure (EXAFS) spectroscopy to interpret the influence of NTE on the temperature dependence of infrared absorption spectra of ${\mathrm{ScF}}_{3}$. Original infrared absorption and EXAFS experiments in a large temperature range are presented and interpreted using ab initio lattice dynamics simulations within and beyond quasiharmonic approximations. We demonstrate that ab initio e…

Oxygen vacancy formation energies in Sr-doped complex perovskites: ab initio thermodynamic study

Abstract La 1 − x Sr x Co 0.25 Fe 0.75 O 3 − δ is known as one of the best cathode materials for permeation membranes and solid oxide fuel cells. Optimization of its chemical composition is a challenging problem. One of the key properties is concentration of oxygen vacancies, which is controlled by their formation energies. Ab initio calculations were employed in order to study the formation of oxygen vacancies in La 1 − x Sr x Co 0.25 Fe 0.75 O 3 − δ perovskites by varying the Sr content from x = 12.5% to 50%. The formation energies were obtained for different stoichiometries as functions of temperature and oxygen partial pressure. For this purpose we calculated the phonon frequencies in t…

First-principles LCAO study of phonons in NiWO4

Abstract The electronic, structural and phonon properties of antiferromagnetic wolframite-type NiWO4 have been studied using first-principles spin-polarized LCAO calculations based on the hybrid Hartree-Fock (HF)/density functional (DFT) scheme. The influence of different percentages of HF contribution, i.e. different correlation strength, on the structure and phonon frequencies has been investigated and compared with the available experimental data.

Comparative density-functional LCAO and plane-wave calculations ofLaMnO3surfaces

We compare two approaches to the atomic, electronic, and magnetic structures of LaMnO3 bulk and the (001), (110) surfaces—hybrid B3PW with optimized LCAO basis set (CRYSTAL-2003 code) and GGA-PW91 with plane-wave basis set (VASP 4.6 code). Combining our calculations with those available in the literature, we demonstrate that combination of nonlocal exchange and correlation used in hybrid functionals allows to reproduce the experimental magnetic coupling constants Jab and Jc as well as the optical gap. Surface calculations performed by both methods using slab models show that the antiferromagnetic (AF) and ferromagnetic (FM) (001) surfaces have lower surface energies than the FM (110) surfac…

Theoretical modeling of antiferrodistortive phase transition forSrTiO3ultrathin films

Combining group-theoretical analysis and first-principles density functional theory calculations, we confirm theoretically the antiferrodistortive phase transition in ultrathin SrTiO${}_{3}$ (001) TiO${}_{2}$-terminated films and compare it with a similar transition in the bulk. We demonstrate phonon softening at the $M$ point of the surface Brillouin zone and analyze the change in the calculated electronic and phonon properties upon phase transition.

Interpretation of EXAFS in ReO3using molecular dynamics simulations

Temperature dependent Re L3-edge EXAFS signals from perovskite-type cubic ReO3 have been successfully interpreted using a combination of classical NVT molecular dynamics (MD) and ab initio multiple-scattering approach. The force field model, required for MD simulations, has been determined by fitting the Re-O and O-O pairwise interatomic potentials to a set of experimental data (lattice parameter, elastic constants and bulk modulus) and phonon frequencies, theoretically calculated from the first principles at high symmetry points of the Brillouin zone. The MD simulations reproduce well the anisotropy of thermal vibration for oxygen atoms and confirm a deviation of the mean Re-O-Re angle fro…

Simulation of Young’s moduli for hexagonal ZnO [0 0 0 1]-oriented nanowires: first principles and molecular mechanical calculations

The authors thank A Gulans, B Polyakov and S Vlassov for stimulating discussions. This study has been supported by the ERA.Net RUS Plus project No. 237 Watersplit. AB, RE and SL acknowledge the financial support by the Russian Foundation for Basic Research (Grant No. 17-03-00130-a) and the assistance of the Saint Petersburg State University Computer Center in the accomplishment of high-performance computations.

Large-scale ab initio modelling of defects in perovskites: Fe impurity in SrTiO3

Abstract Using the unrestricted Hartree–Fock method and supercells containing up to 160 atoms, we calculated the energy level positions in the gap and atomic geometry for the Fe4+ impurity substituting for a host Ti atom in SrTiO3. In agreement with experiment, the high-spin (S=2) state is much lower in energy than the zero-spin state. The energy level positions strongly depend on the asymmetric displacement mode of the six nearest O ions which is a combination of the Jahn–Teller and breathing modes. A considerable covalent bonding between the Fe ion and the four nearest O ions takes place.

Antiferromagnetism-Induced Spin Splitting in Systems Described by Magnetic Layer Groups

Phonon calculations in cubic and tetragonal phases of SrTiO3: A comparative LCAO and plane-wave study

The atomic, electronic structure and phonon frequencies have been calculated in cubic and low-temperature tetragonal SrTiO${}_{3}$ phases at the ab initio level. We demonstrate that the use of the hybrid exchange-correlation PBE0 functional gives the best agreement with experimental data. The results for the standard generalized gradient approximation (PBE) and hybrid PBE0 functionals are compared for the two types of approaches: a linear combination of atomic orbitals (CRYSTAL09 computer code) and plane waves (VASP5.2 code). The relation between cubic and tetragonal phases and the relevant antiferrodistortive phase transition is discussed in terms of group theory and is illustrated with an…

Energetic stability and photocatalytic activity of SrTiO3 nanowires: ab initio simulations

First principles periodic calculations based on the density functional theory within the localized atomic orbital approach (DFT-LCAO) using the hybrid exchange–correlation potential PBE0 have been performed in order to simulate the structural and electronic properties of both stoichiometric and nonstoichiometric [001]-oriented four-faceted SrTiO3 (STO) nanowires (NW) of cubic structure. Their diameters have been varied from 0.3 up to 2.4 nm with a corresponding consequent change of NW cross-section from 2 � 2t o 5� 5 extension of the lattice constant in bulk. Energetic stability of STO NW (both stoichiometric and non-stoichiometric) has been found to increase with the decrease of their form…

Symmetry and models of single-walled TiO2 nanotubes with rectangular morphology

Abstract The formalism of line symmetry groups for one-periodic (1D) nanostructures with rotohelical symmetry has been applied for symmetry analysis of single-walled titania nanotubes (SW TiO2 NTs) formed by rolling up the stoichiometric two-periodic (2D) slabs of anatase structure. Either six- or twelve-layer (101) slabs have been cut from TiO2 crystal in a stable anatase phase. After structural optimization, the latter keeps the centered rectangular symmetry of initial slab slightly compressed along a direction coincided with large sides of elemental rectangles. We have considered two sets of SW TiO2 NTs with optimized six- and twelve-layer structures, which possess chiralities (−n, n) an…

First-principles calculations on double-walled inorganic nanotubes with hexagonal chiralities

The two sets of commensurate double-walled boron nitride and titania hexagonally-structured nanotubes (DW BN and TiO2 NTs) possessing either armchair- or zigzag-type chiralities have been considered, i.e., (n1,n1)@(n2,n2) or (n1,0)@(n2,0), respectively. For symmetry analysis of these nanotubes, the line symmetry groups for one-periodic (1D) nanostructures with rotohelical symmetry have been applied. To analyze the structural and electronic properties of hexagonal DW NTs, a series of large-scale ab initio DFT-LCAO calculations have been performed using the hybrid Hartree-Fock/Kohn-Sham exchange-correlation functional PBE0 (as implemented in CRYSTAL-09 code). To establish the optimal inter-sh…

Doped 1D Nanostructures of Transition-metal Oxides: First-principles Evaluation of Photocatalytic Suitability

DFT plane wave calculations of the atomic and electronic structure of LaMnO3(001) surface

We present the results of ab initio DFT plane wave periodic structure calculations of the LaMnO3 (001) surface. The effects related to three different kinds of pseudopotentials, the slab thickness, magnetic ordering, and surface relaxation are studied and discussed. The antiferromagnetic surface lowest in energy (that is, the spins on Mn ions are parallel in basal plane and antiparallel from plane to plane) has a considerable atomic relaxation up to the fourth plane from the surface. The calculated (Bader) effective charges and the electronic density maps demonstrate a considerable reduction of the Mn atom ionicity on the surface accompanied by a covalent contribution to the Mn–O bonding.