Suchergebnisse

This research was funded by the Latvian Council of Science (under the grant project lzp-2018/1-0147). Authors thank W. Chueh, J. Serra, R. Merkle, A. Popov for fruitful discussions. ; The atomic and electronic structure of CeO2 doped with Tb has been calculated from first principles with inclusion of strong correlation effects on the basis of Hubbard model (DFT + U). The two values of Hubbard U-parameter were applied separately on Ce and Tb ions, in order to treat correctly two oxidation states of Tb (3 + and 4+). Crystal structure distortion is also discussed for Tb3+ ions in ceria without oxygen vacancies. The corresponding total energy difference between the 3 + and 4 + states is very small and, thus, these states can co-exist without oxygen vacancy formation (unlike Gd doping). Multiple configurations have been obtained with localization of electrons on different number of cations, if the Tb ion has an oxygen vacancy nearby. A site symmetry approach has been successfully applied to identify the ground state configuration. Gibbs formation energy of oxygen vacancy due to Tb doping is reduced by almost a factor of four, in comparison with the pure CeO2. The dependence of Gibbs formation energy on the temperature and oxygen partial pressure is discussed. It has been also shown that the lowest formation energy for the small polaron occurs when the Ce3+ and Tb3+ ions are located as nearest neighbors to oxygen vacancy. The results obtained are compared with the existing literature data from the electrical conductivity and optical measurements. ; Latvian Council of Science grant project lzp-2018/1-0147; 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

This research is funded by the Latvian Council of Science, project No. lzp-2018/1-0147. The computer resources were provided by Stuttgart Supercomputing Center (project DEFTD 12939) and Latvian Super Cluster (LASC). Many thanks to R. Dovesi, A. Erba, and M. Rérat for numerous stimulating discussions. ; We present the results of a detailed first principles study of the piezoelectric properties of the (SrTiO3)m/(BaTiO3)M−m heterostructure using the 3D STOm/BTOM−m superlattice model. The atomic basis set, hybrid functionals and slabs with different numbers of STO and BTO layers were used. The interplay between ferroelectric (FEz) and antiferrodistortive (AFDz) displacements is carefully analyzed. Based on the experimental data and group theoretical analysis, we deduce two possible space groups of tetragonal symmetry which allow us to reproduce the experimentally known pure STO and BTO bulk phases in the limiting cases, and to model the corresponding intermediate superlattices. The characteristic feature of the space group P4mm (#99) model is atomic displacements in the [001] direction, which allows us to simulate the FEz displacements, whereas the P4 (#75) model besides FEz displacements permits oxygen octahedra antiphase rotations around the [001] direction and thus AFDz displacements. Our calculations demonstrate that for m/M ≤ 0.75 layer ratios both models show similar geometries and piezoelectric constants. Moreover, both models predict an approximately 6-fold increase of the piezoelectric constant e33 compared to the BaTiO3 bulk value, albeit at slightly different layer ratios. The obtained results clearly demonstrate that piezoelectricity arises due to the coordinated collective FEz displacements of atoms in both STO and BTO slabs and interfaces and reaches its maximum when the superlattice approaches the point where the tetragonal phase becomes unstable and transforms to a pseudo-cubic phase. We demonstrate that even a single or double layer of BTO is sufficient to trigger FEz displacements in the STO slab, in P4mm and P4 models, respectively. ; Latvian Council of Science, project No. lzp-2018/1-0147; Stuttgart Supercomputing Center project DEFTD 12939; Latvian Super Cluster LASC; 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

We have performed this work within the framework of the EUROfusion Consortium receiving funding from the European grant agreement 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Authors thank R. Vila, A.I. Popov, A. Luchshik and R.A. Evarestov for fruitful discussions. To carry out large-scale calculations, we have used the HPC supercomputer at Stuttgart University (Germany) ; We have calculated possible migration trajectories for single-charged interstitial Oi− anion using large-scale hybrid density functional theory within linear combination of atomic orbitals approach to defective α-Al2O3 crystals. The most energetically favorable configuration for charged Oi− anion is formation of pseudo-dumbbell (split interstitial) with a regular Oreg ion. For charged interstitial oxygen migration, the energy barrier turns out to be ∼0.8–1.0 eV. This is considerably smaller than that for a neutral interstitial atoms (1.3 eV), in agreement with experimental data. ; EUROfusion Consortium receiving funding from the European grant agreement 633053; 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

This study has been carried out within the framework of the EURO fusion Consortium and has been provided funding from the Euratom research and training program 2014–2018 under grant agreement No. 633053. The authors are indebted to A.I. Popov, A.C. Lushchik and R. Vila for stimulating discussions. Technical assistance from O. Lisovski is appreciated too. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Calculations have been performed using Marconi supercomputer system based in Italy at CINECA Supercomputing Centre. ; Thermal stability of the primary electronic defects – F‐type centers – in oxide materials is controlled by their recombination with much more mobile complementary defects – interstitial oxygen ions Oi. Thus, the study of interstitial ion migration is of key importance for the prediction of radiation damage in oxides. In this study, several possible migration trajectories for neutral and charged interstitial oxygen ions are calculated in MgAl2O4 spinel using the first principles calculations of atomic and electronic structure. The lowest energy barriers are ≈1.0–1.1 eV and 0.8 eV, respectively. The effective atomic charges, charge redistribution, and lengths of bonds closest to Oi interstitials are analyzed in detail. ; EURO fusion Consortium Euratom research and training program 2014–2018 under grant agreement No. 633053; 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

This research was partly funded by the Russian Science Foundation (under the project 14-43-0005) and ERA-NET HarvEnPiez project, with the computer resources provided by Stuttgart Supercomputing Centre (Project DEFTD 12939). A. C. also acknowledges financial support from the University of Latvia Foundation (Arnis Riekstins's ''MikroTik'' donation). Authors thank R. Merkle, A. Popov for fruitful discussions. ; Tb-doped CeO2 (ceria) is a promising mixed conductor for oxygen permeation membranes and reversible oxygen sorbents. To predict solubility of Tb ions in ceria for a wide range of concentrations, density functional theory (DFT+U) calculations with two different values of Hubbard U-parameter on Tb and Ce ions were combined with alloy thermodynamics and the Concentration Wave approach. It is shown that, to predict properties of disordered solid solutions at finite temperatures, the energy parameters in the mixing energies can be extracted from the DFT+U calculations performed at T = 0 K for two ordered configurations of the dopant in the supercells. The unlimited solubility of Tb4+ in CeO2 in the quasi-binary cross-section CeO2-TbO2 is predicted in the temperature range where both stoichiometric TbO2 and CeO2 reveal fluorite structures (above 700 °C). ; Russian Science Foundation (under the project 14-43-0005); ERA-NET HarvEnPiez project; Stuttgart Supercomputing Centre Project DEFTD 12939; 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

Financial support by the German–Israeli Foundation for Scientific Research and Development (grant I-1342-302.5/2016) and the Latvian Council of Science (grant lzp-2018/1-0147 (D.G., E.A.K.)) is gratefully acknowledged. The authors further thank Guntars Zvejnieks for help with CRYSTAL code calculations. ; Protonic-electronic mixed-conducting perovskites are relevant as cathode materials for protonic ceramic fuel cells (PCFCs). In the present study, the relation between the electronic structure and the thermodynamics of oxygen nonstoichiometry and hydration is investigated for BaFeO3-δ and Ba0.5Sr0.5FeO3-δ by means of density functional theory. The calculations are performed at the PBE + U level and yield ground-state electronic structures dominated by an oxygen-to-metal charge transfer with electron holes in the O 2p valence bands. Oxygen nonstoichiometry is modeled for 0 ≤ δ≤ 0.5 with oxygen vacancies in doubly positive charge states. The energy to form an oxygen vacancy is found to increase upon reduction, i.e., decreasing concentration of ligand holes. The higher vacancy formation energy in reduced (Ba,Sr)FeO3-δ is attributed to a higher Fermi level at which electrons remaining in the lattice from the removed oxide ions have to be accommodated. The energy for dissociative H2O absorption into oxygen vacancies is found to vary considerably with δ, ranging from ≈-0.2 to ≈-1.0 eV in BaFeO3-δ and from ≈0.2 to ≈-0.6 eV in Ba0.5Sr0.5FeO3-δ. This dependence is assigned to the annihilation of ligand holes during oxygen release, which leads to an increase in the ionic charge of the remaining lattice oxide ions. The present study provides sound evidence that p-type electronic conductivity and the susceptibility for H2O absorption are antagonistic properties since both depend in opposite directions on the concentration of ligand holes. The reported trends regarding oxygenation and hydration energies are in line with the experimental observations. ; Latvian Council of Science lzp-2018/1-0147; German–Israeli Foundation for Scientific Research and Development grant I-1342-302.5/2016; 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

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. ; Using site symmetry analysis, four possible positions of interstitial oxygen atoms in the α-Al2O3 hexagonal structure have been identified and studied. First principles hybrid functional calculations of the relevant atomic and electronic structures for interstitial Oi atom insertion in these positions reveal differences in energies of ∼1.5 eV. This approach allows us to get the lowest energy configuration, avoiding time-consuming calculations. It is shown that the triplet oxygen atom is barrierless displaced towards the nearest regular oxygen ion, forming a singlet dumbbell (split interstitial) configuration with an energy gain of ∼2.5 eV. The charge and spatial structure of the dumbbell is discussed. Our results are important, in particular, for understanding the radiation properties and stability of α-Al2O3 and other oxide crystals. ; EUROfusion Consortium European Union's Horizon 2020 Research and Innovation Programme under grant agreement 633053; Latvian National Research Program IMIS2; 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

This work has been performed within the framework of the EUROfusion Enabling Research project: ENR-MFE19.ISSP-UL-02 "Advanced experimental and theoretical analysis of defect evolution and structural disordering in optical and dielectric materials for fusion application". The views and opinions expressed herein do not necessarily reflect those of the European Commission. ; We have performed the density functional calculations (DFT) on the hole-type defects (V-centres) in magnesium aluminate spinel (MgAl2O4) following the results of recent paramagnetic resonance measurements (EPR) in Nucl. Inst. Methods Phys. Res. B 435 (2018) 31–37. The hybrid B3LYP functional calculations using large supercells of 448 atoms have demonstrated excellent results not only for bulk properties but also properties of the V-centres in MgAl2O4. Three types of V-centres have been considered and confirmed, namely V1, V2 and V22. The DFT calculations have revealed the atomic relaxation pattern and spin density distribution around the hole-type defects that is suggested as an important complement to the experiments. Moreover, the calculated hyperfine coupling constants (HCCs) have been analyzed and compared with those from the measured EPR spectra. A good agreement between the calculated and measured HCC values is observed and discussed. ; EUROfusion Enabling Research project: ENR-MFE19.ISSP-UL-02; 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

This research was partly supported by the Kazakhstan Science Project № AP05134367«Synthesis of nanocrystals in track templates of SiO2/Si for sensory, nano- and optoelectronic applications», as well as by Latvian Research Council project lzp-2018/1-0214. Calculations were performed on Super Cluster (LASC) in the Institute of Solid State Physics (ISSP) of the University of Latvia. Authors are indebted to S. Piskunov for stimulating discussions. ; Ab initio calculations of the atomic, electronic and vibrational structure of a pure and Co+2 doped MgF2 crystals were performed and discussed. We demonstrate that Co+2 (3d7) ions substituting for Mg is in the high spin state. In particular, the role of exact non-local exchange is emphasized for a proper reproduction of not only the band gap but also other MgF2 bulk properties. It allows us for reliable estimate of the dopant energy levels position in the band gap, and its comparison with the experimental data. Thus, the present ab initio calculations and experiment data demonstrate that the Co+2 ground state level lies at ≈2 eV above the valence band top. ; Kazakhstan Science Project № AP05134367; Latvian Council of Science project lzp-2018/1-0214; 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

We greatly acknowledge the financial support via the ERAF Project No. 1.1.1.1/18/A/073. Calculations have been performed under the Project HPC-EUROPA3 (INFRAIA-2016-1-730897), with the support of the EC Research Innovation Action under the H2020 Programme. A. C. gratefully acknowledges the technical support received from KTH-PDC. 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 ; We combined the hybrid density functional theory (DFT) calculations and X-ray absorption spectroscopy (XAS) experiments in the study of the local atomic structure around Ir ions in ZnO thin films with different iridium content. This was then used in the first principles analysis of the thermoelectric properties of material. The emphasis has been put on the conditions for a positive Seebeck coefficient and p-type electrical conductivity as the functions of the Fermi level. We studied both computationally and experimentally several possible IrOx polyhedra (complexes) with a different number of surrounding oxygens and Ir oxidation states, including those with the formation of peroxide ions (O22−). In particular, octahedral coordination of iridium ions was identified by reverse Monte Carlo (RMC) simulations of the Ir L3-edge EXAFS spectra of ZnO:Ir thin films as the predominant complex, which is supported by the calculated lowest interstitial oxygen incorporation energies. All the calculated IrOx (x = 4, 5, 6) complexes, regardless of Ir the oxidation state, demonstrate potential for p-type conduction if the Fermi level (μF) falls in the range of 0–0.8 eV from the valence band maximum (VBM) and the Ir concentration is high enough (12.5% in the present DFT calculations). Even though the corresponding calculated Seebeck coefficient (S) around 80–89 μV K−1 slightly exceeds the experimental values, we emphasise the presence of an important plateau in the dependence of S on μF in this range for two complexes with the formation of peroxide ions (O22−). We predicted also that peroxide ions O22− are characterized by the calculated phonon frequencies of 810–942 cm−1 in agreement with our previous Raman experimental results. In this light, we discuss the high sensitivity of calculated S(μF) dependences to the atomic and electronic structure.--//--This is the preprint version of the following article: Andrei Chesnokov, Denis Gryaznov, Natalia V. Skorodumova, Eugene A. Kotomin, Andrea Zitolo, Martins Zubkins, Alexei Kuzmin, Andris Anspoks, Juris Purans; The local atomic structure and thermoelectric properties of Ir-doped ZnO: hybrid DFT calculations and XAS experiment, Journal of Materials CHemistry C: Volume 14, 2021, which has been published in final form at https://pubs.rsc.org/en/content/articlelanding/2021/tc/d1tc00223f#!divAbstract This article may be used for non-commercial purposes in accordance with Royal Society of Chemistry Terms and Conditions for Sharing and Self-Archiving. ; ERAF Project No. 1.1.1.1/18/A/073; Project HPC-EUROPA3 (INFRAIA-2016-1-730897) under EC Research Innovation Action under H2020; 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

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under Grant Agreement No. 633053 and Enabling Research project: ENR-MFE19.ISSP-UL-02 "Advanced experimental and theoretical analysis of defect evolution and structural disordering in optical and dielectric materials for fusion application". The views and opinions expressed herein do not necessarily reflect those of the European Commission. In addition, the research leading to these results has received funding from the Estonian Research Council grant (PUT PRG619). ; A single radiation-induced superoxide ion O2- has been observed for the first time in metal oxides. This structural defect has been revealed in fast-neutron-irradiated (6.9×1018n/cm2) corundum (α-Al2O3) single crystals using the EPR method. Based on the angular dependence of the EPR lines at the magnetic field rotation in different planes and the determined g tensor components, it is shown that this hole-type O2- center (i) incorporates one regular and one interstitial oxygen atoms being stabilized by a trapped hole (S = 1/2), (ii) occupies one oxygen site in the (0001) plane being oriented along the a axis, and (iii) does not contain any other imperfection/defect in its immediate vicinity. The thermal stepwise annealing (observed via the EPR signal and corresponding optical absorption bands) of the O2- centers, caused by their destruction with release of a mobile ion (tentatively the oxygen ion with the formal charge −1), occurs at 500–750 K, simultaneously with the partial decay of single F-type centers (mostly with the EPR-active F+ centers). The obtained experimental results are in line with the superoxide defect configurations obtained via density functional theory (DFT) calculations employing the hybrid B3PW exchange-correlation functional. In particular, the DFT calculations confirm the O2- center spin S = 1/2, its orientation along the a axis. The O2- center is characterized by a short O–O bond length of 1.34 Å and different atomic charges and magnetic moments of the two oxygens. We emphasize the important role of atomic charges and magnetic moments analysis in order to identify the ground state configuration. ; Eesti Teadusagentuur PUT PRG619; H2020 Euratom ENR-MFE19,633053; 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