The first-principles treatment of the electron-correlation and spin-orbital effects in uranium mononitride nuclear fuels

The DFT+U calculations were employed in a detailed study of the strong electron correlation effects in promising nuclear fuel -- uranium mononitride (UN). A simple method for solving the multiple minima problem in DFT+U simulations and insure obtaining the correct ground state is suggested and applied. The crucial role of spin-orbit interactions in reproduction of the U atom total magnetic moment is demonstrated. Basic material properties (the lattice constants, the spin- and total magnetic moments on U atoms, magnetic ordering, and the density of states) were calculated varying the Hubbard U-parameter. Varying the tetragonal unit cell distortion, the meta-stable states have been carefully identified and analyzed. The difference of the magnetic and structural properties obtained for the meta-stable and ground states are discussed. The optimal effective Hubbard parameter Ueff =1.85 eV reproduces correctly the UN anti-ferromagnetic ordering, and only slightly overestimates the experimental total magnetic moment of U atom and the unit cell volume.JRC.E.3-Materials researc

First principles calculations on CeO2 doped with Tb3+ ions

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 CAMART

Ab initio simulations on charged interstitial oxygen migration in corundum

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 CAMART

Dopant solubility in ceria: alloy thermodynamics combined with the DFT+U calculations

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 CAMART

Interface-induced enhancement of piezoelectricity in the (SrTiO 3 ) m /(BaTiO 3 ) M−m superlattice for energy harvesting applications

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 CAMART

Peculiarities of Phase Formation in Mn-Based Na SuperIonic Conductor (NaSICon) Systems: The Case of Na1+2 xMnxTi2- x(PO4)3(0.0 ≤ x ≤ 1.5)

This project has received funding from the European Regional Development Fund (Project no. 01.2.2-LMT-K-718-02–0005) under grant agreement with the Research Council of Lithuania (LMTLT). We thank the High Performance Computing Center “HPC Saulėtekis” at the Faculty of Physics, Vilnius University, for the use of computational resources.NAtrium SuperIonic CONductor (NASICON) structured phosphate framework compounds are attracting a great deal of interest as suitable electrode materials for "rocking chair"type batteries. Manganese-based electrode materials are among the most favored due to their superior stability, resource non-criticality, and high electrode potentials. Although a large share of research was devoted to Mn-based oxides for Li- and Na-ion batteries, the understanding of thermodynamics and phase formation in Mn-rich polyanions is still generally lacking. In this study, we investigate a bifunctional Na-ion battery electrode system based on NASICON-structured Na1+2xMnxTi2-x(PO4)3 (0.0 ≤ x ≤ 1.5). In order to analyze the thermodynamic and phase formation properties, we construct a composition-temperature phase diagram using a computational sampling by density functional theory, cluster expansion, and semi-grand canonical Monte Carlo methods. The results indicate finite thermodynamic limits of possible Mn concentrations in this system, which are primarily determined by the phase separation into stoichiometric Na3MnTi(PO4)3 (x = 1.0) and NaTi2(PO4)3 for x 1.0. The theoretical predictions are corroborated by experiments obtained using X-ray diffraction and Raman spectroscopy on solid-state and sol-gel prepared samples. The results confirm that this system does not show a solid solution type behavior but phase-separates into thermodynamically more stable sodium ordered monoclinic α-Na3MnTi(PO4)3 (space group C2) and other phases. In addition to sodium ordering, the anti-bonding character of the Mn-O bond as compared to Ti-O is suggested as another important factor governing the stability of Mn-based NASICONs. We believe that these results will not only clarify some important questions regarding the thermodynamic properties of NASICON frameworks but will also be helpful for a more general understanding of polyanionic systems. ©ERDF (Project no. 01.2.2-LMT-K-718-02–0005); The Institute of Solid State Physics, University of Latvia (Latvia), as the Centre of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-Teaming Phase2 under grant agreement No. 739508, project CAMART2

Interdependence of Oxygenation and Hydration in Mixed-Conducting (Ba,Sr)FeO3-δPerovskites Studied by Density Functional Theory

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 CAMART

Proton uptake in the mixed ionic and electronic conductors Ba1-xSrxFeO3-d

Cathode materials for proton-conducting ceramic fuel cells (PCFC) should combine electronic conductivity with adequate proton conductivity and thereby extend the water formation process from the triple phase boundary to the entire surface of the porous cathode. A variety of such materials including perovskite-structured Ba0.95La0.05FeO3 has been studied experimentally with regard to proton uptake, revealing a systematically lower proton concentration than in electrolyte materials and a peculiar interaction between electronic charge carriers (i.e. holes) and ionic charge carriers (i.e. protons). [1] Please click Additional Files below to see the full abstract