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

First-Principles Modelling of N-Doped Co3O4

The project Nr. AP05131211 “First Principles Investigation on Catalytic Properties of N-doped Co3O4” is supported by the Ministry of Education and Science of the Republic of Kazakhstan within the framework of the grant funding for scientific and (or) scientific and technical research for 2018-2020. The authors thank T. Inerbaev and A. Popov for fruitful discussions and valuable suggestions. Yu.M. thanks M.Putnina for the technical assistance in preparation of the manuscript.N-doped Co3O4 is a promising electrocatalyst. By means of first-principles calculations, various concentrations and spatial arrangements of NO atoms were modelled. Mutual interaction of the dopant atoms was analysed with respect to single NO atom. Charge redistribution, caused by doping, was calculated.Ministry of Education and Science of the Republic of Kazakhstan No. AP05131211; 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

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

Selective uptake and desorption of carbon dioxide in carbon honeycombs of different sizes

The EK research was performed in the Center of Excellence of the Institute of Solid State Physics, University of Latvia, supported through European Unions Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under Grant Agreement No. 739508, Project CAMART2.Carbon honeycombs (CHs) are new carbon cellular structures, very promising in many respects, in particular, for high-capacity storage of various materials, especially in gaseous and liquid forms. In this study, we report a strong uptake of carbon dioxide kept inside carbon honeycomb matrices up to temperatures about three times higher as compared with CO2 desorption at ≈ 90 K from flat solid surfaces in vacuum where we conduct our high-energy electron diffraction experiments. Desorption of CO2 from CH matrices upon heating exhibits non-monotone behavior, which is ascribed to carbon dioxide release from CH channels of different sizes. It is shown that modeling of CO2 uptake, storage, and redistribution in the thin CH channels of certain types and orientations upon heating can explain experimental observations.--//-- This is an Open Access article N. V. Krainyukova, D. G. Diachenko, E. A. Kotomin; Selective uptake and desorption of carbon dioxide in carbon honeycombs of different sizes. Low Temp. Phys. 1 January 2024; 50 (1): 97–102. https://doi.org/10.1063/10.0023898 published under the CC BY licence.The EK research was performed in the Center of Excellence of the Institute of Solid State Physics, University of Latvia, supported through European Unions Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under Grant Agreement No. 739508, Project CAMART2

Ab initio simulation of (Ba,Sr)TiO3 and (Ba,Ca)TiO3 perovskite solid solutions

This research was supported by the ERA-NET HarvEnPiez project. Many thanks to R. Dovesi, M.M. Kržmanc and D. Gryaznov for fruitful discussions.The results of ab initio (first-principles) computations of structural, elastic and piezoelectric properties of Ba(1−x)SrxTiO3 (BSTO) and Ba(1−x)CaxTiO3 (BCTO) perovskite solid solutions are presented, discussed and compared. Calculations are performed with the CRYSTAL14 computer code within the linear combination of atomic orbitals (LCAO) approximation, using advanced hybrid functionals of the density-functional-theory (DFT). Supercell model allows us to simulate solid solutions with different chemical compositions (x = 0, 0.125 and 0.25) within ferroelectric tetragonal phases (x < 0.3) of both solid solutions. It is shown that configurational disorder has to be taken into account in simulations of BCTO solid solutions, while for BSTO this effect is rather small. Both BSTO and BCTO show significantly enhanced piezoelectric properties, in a comparison with pure BaTiO3. However, these solid solutions demonstrate opposite behaviour of a tetragonal ratio c/a and elastic constants as the functions of chemical composition. It is predicted that due to decrease of the elastic constants in BCTO, it has much higher converse piezoelectric constants than BSTO.ERA-NET HarvEnPiez project; 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

Kinetics of dimer F2 type center annealing in MgF2 crystals

Authors are greatly indebted to V. Lisitsyn, A. Ch. Lushchik and R.Vila for stimulating discussions. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement number 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. The calculations were performed using facilities of the Stuttgart Supercomputer Center (project DEFTD 12939).In this paper, we analyzed experimental annealing kinetics of the primary electronic F centers and dimer F2 centers observed in MgF2 at higher radiation doses and temperatures. The developed phenomenological theory takes into account the interstitial ion diffusion and recombination with the F2-centers, as well as mutual sequential transformation with temperature growth of three types of experimentally observed dimer centers: F2(1), F2(2), F2(3) (which differ tentatively by charges (0, +1, +2) with respect to the host crystalline sites). The results of the electron, neutron and ion irradiation are compared. As the result, the relative initial concentrations of three types of F2 electronic defects before annealing are obtained, along with energy barriers between their ground states as well as the relaxation energies.European Union’s Horizon 2020 agreement number 633053; Stuttgart Supercomputer Center 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

Analysis of self-trapped hole mobility in alkali halides and metal halides

Support from Latvian National Research Program IMIS2 (2014–2017) and LZP Grant No. 237/2012 (2013–2016) is greatly appreciated.The small radius hole polarons (self-trapped holes (STH) known also as the Vk centers) are very common color centers observed in numerous alkali halides and alkaline-earth halides. Their mobility controls the rate of secondary reactions between electron and hole defects and thus radiation stability/sensitivity of materials. We have analysed here the correlation between the temperatures at which hole polarons start migration in a series of alkali halides (fluorites, chlorides, bromides, iodides) and the lattice displacement around quasi-molecule. These results are especially important for identification of the self-trapped holes, for example, in novel scintillating materials such as SrI2, as well as in a large family of perovskite halides and more complex halide materials.IMIS2; LZP Grant No. 237/2012; 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

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.We present here first principles hybrid functional calculations of the atomic and electronic structure of several iodine-related point defects in CsPbI3, a material relevant for photovoltaic applications. We show that the presence of neutral interstitial I atoms or electron holes leads to the formation of di-halide dumbbells of I2− (analogous to the well-known situation in alkali halides). Their formation and one-electron energies in the band gap are determined. The formation energy of the Frenkel defect pair (I vacancies and neutral interstitial I atoms) is found to be ∼1 eV, and as such is smaller than the band gap. We conclude that both iodine dumbbells and iodine vacancies could be, in principle, easily produced by interband optical excitation.Latvian Council for Science (grant LZP-2018/1-0147 to EK); 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