Density functional simulation of the BaZrO3 (011) surface structure

The atomic structure and charge redistribution of different terminations of BaZrO3 (011) surfaces have been studied using density functional simulations. We found that the O-terminated (011) flat surface had the smallest cleavage energy among (011) surfaces, but this value was still twice as large as for the formation of a pair of complimentary (001) surfaces. The density functional calculations allowed us to estimate the excess surface Gibb's free energy and to compare stability of different (011) surfaces as a function of chemical environment. In addition, we compared stability of BaZrO3 (011) surfaces with respect to BaZrO3 (001) surfaces. Within boundaries, where BaZrO3 does not decompose, only the Ba- and O-terminated (011) surfaces appeared to be stable. However, if (001) surfaces are also taken into consideration, the BaO-terminated (001) surface is the only stable surface among all considered (001) and (011) surfaces

Atomistic simulation of SrTiO3(001) surface relaxation

The (001) surface relaxation of the cubic perovskite SrTiO3 crystal has been studied using the shell model. The positions of atoms in several surface layers embedded in the electrostatic field of the remainder of the crystal are calculated. We show that Ti4+, Sr2+ and O2- ions in six near-surface layers are displaced differently from their crystalline sites which leads to the creation of so-called surface rumpling, a dipole moment, and an electric field in the near-surface region. Calculated atomic displacements are compared with LEED experimental data

Thermodynamic stability of non-stoichiometric SrFeO 3-δ : a hybrid DFT study

SrFeO3-δ is mixed ionic-electronic conductor with complex magnetic structure which reveals also colossal magnetoresistance effect. This material and its solid solutions are attractive for various spintronic, catalytic and electrochemical applications, including cathodes for solid oxide fuel cells and permeation membranes. Its properties strongly depend on oxygen non-stoichiometry. Ab initio hybrid functional approach was applied here for a study of thermodynamic stability of a series of SrFeO3-δ compositions with several non-stoichiometries δ, ranging from 0 to 0.5 (SrFeO3 - SrFeO2.875 - SrFeO2.75 - SrFeO2.5) as the function of temperature and oxygen pressure. The results obtained by considering Fe as all-electron atom and within the effective core potential technique are compared. Based on our calculations, the phase diagrams were constructed allowing the determination of environmental conditions for the existence of stable phases. It is shown that (within an employed model) only the SrFeO2.5 phase appears to be stable. The stability region for this phase is re-drawn at the contour map of oxygen chemical potential, presented as a function of temperature and oxygen partial pressure. A similar analysis is also performed using experimental Gibbs energies of perovskite formation from the elements. The present modelling strongly suggests a considerable attraction between neutral oxygen vacancies. These vacancies are created during a series of above mentioned SrFeO3-δ mutual transformations accompanied by oxygen release.Latvian Council of Science LZP FLPP grant 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

Thermodynamics of ABO3-Type Perovskite Surfaces

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Density functional theory calculations on magnetic properties of actinide compounds

We have performed a detailed analysis of the magnetic (collinear and noncollinear) order and atomic and the electron structures of UO2, PuO2 and UN on the basis of density functional theory with the Hubbard electron correlation correction (DFT+U). We have shown that the 3-k magnetic structure of UO2 is the lowest in energy for the Hubbard parameter value of U=4.6 eV (and J=0.5 eV) consistent with experiments when Dudarev's formalism is used. In contrast to UO2, UN and PuO2 show no trend for a distortion towards rhombohedral structure and, thus, no complex 3-k magnetic structure is to be anticipated in these materials.Comment: 5 pages, 3 figures 1 table, submitted to Phys. Chem. Chem. Phy

Density functional calculations of extended, periodic systems using Coulomb corrected molecular fractionation with conjugated caps method (CC-MFCC)

A consistent DFT based formulation of the order-N molecular fractionation with conjugated caps method in which a molecular system is calculated considering a set of finite fragments, is proposed. Here we apply the method and test its performance on a periodic metal–organic framework system.</p

Thermodynamic stability of stoichiometric LaFeO3 and BiFeO3: A hybrid DFT study

The authors are greatly indebted to R. Dovesi, R. Orlando, R. Merkle, and J. Serra for many stimulating discussions. E. H. thanks also the Department of Physical Chemistry of the Max Planck Institute for Solid State Research for long-term hospitality and support. This study was partly supported by the EC GREEN-CC FP7 project 608524. E. H., E. A. K. and A. A. B. acknowledge also the Russian Science Foundation for provided financial support through funding under the project 14-43-00052 for the analysis of the experimental literature on complex perovskite formation enthalpies, and the program of National Research Nuclear University "MEPhI" on improving the scientific competitiveness (A. A. B). Calculations were performed at the High Performance Computer Center in Stuttgart (HLRS, project DEFTD 12939) and in National Research Nuclear University MEPhI, Moscow, Russia.BiFeO3 perovskite attracts great attention due to its multiferroic properties and potential use as a parent material for Bi1-xSrxFeO3-δ and Bi1-xSrxFe1-yCoyO3-δ solid solutions in intermediate temperature cathodes of oxide fuel cells. Another iron-based LaFeO3 perovskite is the end member for well-known solid solutions (La1-xSrxFe1-yCoyO3-δ) used for oxide fuel cells and other electrochemical devices. In this study an ab initio hybrid functional approach was used for the study of the thermodynamic stability of both LaFeO3 and BiFeO3 with respect to decompositions to binary oxides and to elements, as a function of temperature and oxygen pressure. The localized (LCAO) basis sets describing the crystalline electron wave functions were carefully re-optimized within the CRYSTAL09 computer code. The results obtained by considering Fe as an all-electron atom and within the effective core potential technique are compared in detail. Based on our calculations, the phase diagrams were constructed allowing us to predict the stability region of stoichiometric materials in terms of atomic chemical potentials. This permits determining the environmental conditions for the existence of stable BiFeO3 and LaFeO3. These conditions were presented as contour maps of oxygen atoms' chemical potential as a function of temperature and partial pressure of oxygen gas. A similar analysis was also performed using the experimental Gibbs energies of formation. The obtained phase diagrams and contour maps are compared with the calculated ones.DEFTD 12939; Russian Science Foundation 14-43-00052; National Research Nuclear University MEPhI; Seventh Framework Programme 608524; European Commission; 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

Chapter Thermodynamics of ABO3-Type Perovskite Surfaces

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