B6 – LU institūti un aģentūras / Institutes and agencies of the UL

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1D polinomiālu funkciju precīzi interpolējoša pikseļa forma
(2013-01-23) Ķikusts, Paulis
2006. gada publiskais pārskats
(LU Latvijas Vēstures institūts, 2007) Latvijas Universitātes aģentūra Latvijas Universitātes Vēstures institūts
2008. gada publiskais pārskats
(LU Latvijas Vēstures institūts, 2009) Latvijas Universitātes aģentūra Latvijas Universitātes Vēstures institūts
2010. gada publiskais pārskats
(LU Latvijas Vēstures institūts, 2011) Latvijas Universitātes aģentūra Latvijas Universitātes Vēstures institūts
20th-century Baltic drama: comparative paradigms
(University of Tartu Press, 2014) Kalnačs, Benedikts
2d slab models of nanotubes based on tetragonal tio2 structures: Validation over a diameter range
(Multidisciplinary Digital Publishing Institute (MDPI), 2021) Lisovski, Oleg; Piskunov, Sergei; Bocharov, Dmitry; Kenmoe, Stephane
One-dimensional nanomaterials receive much attention thanks to their advantageous properties compared to simple, bulk materials. A particular application of 1D nanomaterials is pho-tocatalytic hydrogen generation from water. Such materials are studied not only experimentally, but also computationally. The bottleneck in computations is insufficient computational power to access realistic systems, especially with water or another adsorbed species, using computationally expensive methods, such as ab initio MD. Still, such calculations are necessary for an in-depth understanding of many processes, while the available approximations and simplifications are either not precise or system-dependent. Two-dimensional models as an approximation for TiO2 nanotubes with (101) and (001) structures were proposed by our group for the first time in Comput. Condens. Matter journal in 2018. They were developed at the inexpensive DFT theory level. The principle was to adopt lattice constants from an NT with a specific diameter and keep them fixed in the 2D model optimization, with geometry modifications for one of the models. Our previous work was limited to studying one configuration of a nanotube per 2D model. In this article one of the models was chosen and tested for four different configurations of TiO2 nanotubes: (101) (n,0), (101) (0,n), (001) (n,0), and (001) (0,n). All of them are 6-layered and have rectangular unit cells of tetragonal anatase form. Results of the current study show that the proposed 2D model is indeed universally applicable for different nanotube configurations so that it can be useful in facilitating computationally costly calculations of large systems with adsorbates. © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Published under the CC BY 4.0 license.
2D slab models of TiO2 nanotubes for simulation of water adsorption: Validation over a diameter range
(Elsevier B.V., 2020) Lisovski, Oleg; Piskunov, Sergei; Bocharov, Dmitry; Kenmoe, Stéphane
Currently a lot of attention is paid to 1D nanomaterials due to their advantages in comparison to bulk materials. They offer broad possibilities of application, including photocatalytic water splitting. Simulations of water adsorption on such materials with computationally costly theoretical methods, such as ab initio molecular dynamics, are needed for improvement of such photocatalysts’ efficiency. Still, it is very problematic to treat a real-size nanotube at available computational power. The existing nanotube surface approximations are not accurate and universal enough. We have already proposed methods for 2D model construction out of TiO2 nanotubes of (101) and (001) configuration at the moderately expensive DFT level. The idea behind was to provide a partial description of nanotubular strain by applying lattice constants from nanotubes to slab models, and preserving geometry motifs. We use water adsorption energy, valence band maximum and conduction band minimum positions, as well as DOS shape as criteria for model validation. Our previous work was limited only to specific variants of nanotubes and water adsorption. In this work we establish these novel approaches along a wide nanotube diameter range, in particular for water adsorption studies. We demonstrate that the 2D models do not impose critical compromises in terms of accuracy, and therefore allow calculations of much larger nanotubes than common approaches do.
The 4th international oral history conference «Generation, Migration and Memory: Narrative and its Transformations»: Conference programm and abstracts
(LU Filozofijas un socioloģijas institūts, 2020-11) Latvijas Universitāte, Filozofijas un socioloģijas institūts; Šūpulis, Edmunds; Zirnīte, Māra
The 4th international oral history conference «Generation, Migration and Memory: Narrative and its Transformations», November 12-13, 2020. University of Latvia, Institute of Philosophy and Sociology. Conference programm and abstracts.
Ab initio calculations of CaZrO3 (011) surfaces: systematic trends in polar (011) surface calculations of ABO3 perovskites
(Springer, 2020) Eglitis, Roberts; Kleperis, Janis; Purans, Juris; Popov, Anatoli I.; Jia, Ran
By means of the CRYSTAL computer program package, first-principles calculations of polar ZrO-, Ca- and O-terminated CaZrO3 (011) surfaces were performed. Our calculation results for polar CaZrO3 (011) surfaces are compared with the previous ab initio calculation results for ABO3 perovskite (011) and (001) surfaces. From the results of our hybrid B3LYP calculations, all upper-layer atoms on the ZrO-, Ca- and O-terminated CaZrO3 (011) surfaces relax inwards. The only exception from this systematic trend is outward relaxation of the oxygen atom on the ZrO-terminated CaZrO3 (011) surface. Different ZrO, Ca and O terminations of the CaZrO3 (011) surface lead to a quite different surface energies of 3.46, 1.49, and 2.08 eV. Our calculations predict a considerable increase in the Zr–O chemical bond covalency near the CaZrO3 (011) surface, both in the directions perpendicular to the surface (0.240e) as well as in the plane (0.138e), as compared to the CaZrO3 (001) surface (0.102e) and to the bulk (0.086e). Such increase in the B–O chemical bond population from the bulk towards the (001) and especially (011) surfaces is a systematic trend in all our eight calculated ABO3 perovskites. This work is licensed under a CC BY license.
AB Initio Calculations of CU N @Graphene (0001) Nanostructures for Electrocatalytic Applications
(Sciendo, 2018) Piskunov, Sergei; Zhukovskii, Y.F.; Sokolov, M.N.; Kleperis, Janis
Substitution of fossil-based chemical processes by the combination of electrochemical reactions driven by sources of renewable energy and parallel use of H 2 O and CO 2 to produce carbon and hydrogen, respectively, can serve as direct synthesis of bulk chemicals and fuels. We plan to design and develop a prototype of electrochemical reactor combining cathodic CO 2 -reduction to ethylene and anodic H 2 O oxidation to hydrogen peroxide. We perform ab initio calculations on the atomistic 2D graphene-based models with attached Cu atoms foreseen for dissociation of CO 2 and H 2 O containing complexes, electronic properties of which are described taking into account elemental electrocatalytical reaction steps. The applicability of the model nanostructures for computer simulation on electrical conductivity of charged Cu n /graphene (0001) surface is also reported.
Ab initio calculations of Li2(Co, Mn)O8 solid solutions for rechargeable batteries
(World Scientific Publishing Co. Pte Ltd, 2019) Eglitis, Roberts
Current commercially available rechargeable Li-ion batteries, for example LiCoO2, are working mostly in the 4 V regime. One often suggested possibility to improve the effectivity of Li-ion batteries are the creation of the 5 V cathode materials. We performed quantum mechanical calculations on the average battery voltage for the Li2CoxMn4−xO8 (x = 0, 1, 2, 3 and 4) cathode materials by means of the WIEN2k computer program package. The calculated average battery voltages for x = 0, 1, 2, 3 and 4 are equal to 3.95, 5, 4.47, 4.19 and 3.99 V. Our ab initio calculation results are compared with the available experimental data for x = 0, 1, 2 and 4 which are equal to 4, 5, 5 and 4 V. Thereby, for the Li2Co1Mn3O8 battery cathode material, our calculated average battery voltage around 5 V is in perfect agreement with the experimentally available battery voltage value of 5 Volt. Nevertheless, our calculated average battery voltage is underestimated (4.47 V) for the Li2Co2Mn2O8 cathode material, which also experimentally exhibits the 5 V voltage.
Ab initio calculations of pure and Co+2-doped MgF2 crystals
(Elsevier B.V., 2020) Usseinov, Abay; Gryaznov, Denis; Popov, Anatoli; Kotomin, Eugene A.; Seitov, D. D.; Abuova, Fatima; Nekrasov, Kirill A.; Akilbekov, Abdirash T.
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.
Ab Initio Computations of O and AO as well as ReO2, WO2 and BO2-Terminated ReO3, WO3, BaTiO3, SrTiO3 and BaZrO3 (001) Surfaces
(MDPI, 2022) Eglitis, Roberts I.; Purans, Juris; Popov, Anatoli I.; Bocharov, Dmitry; Chekhovska, Anastasiia; Jia, Ran
We present and discuss the results of surface relaxation and rumpling computations for ReO3, WO3, SrTiO3, BaTiO3 and BaZrO3 (001) surfaces employing a hybrid B3LYP or B3PW description of exchange and correlation. In particular, we perform the first B3LYP computations for O-terminated ReO3 and WO3 (001) surfaces. In most cases, according to our B3LYP or B3PW computations for both surface terminations BO2-and O, AO-terminated ReO3, WO3, BaTiO3, SrTiO3 and BaZrO3 (001) surface upper layer atoms shift downwards, towards the bulk, the second layer atoms shift upwards and the third layer atoms, again, shift downwards. Our ab initio computes that ReO3, WO3, BaTiO3, SrTiO3 and BaZrO3 (001) surface Γ-Γ bandgaps are always smaller than their respective bulk Γ-Γ bandgaps. Our first principles compute that B-O atom chemical bond populations in the BaTiO3, SrTiO3 and BaZrO3 perovskite bulk are always smaller than near their BO2-terminated (001) surfaces. Just opposite, the Re-O and W-O chemical bond populations in the ReO3 (0.212e) and WO3 (0.142e) bulk are slightly larger than near the ReO2 and WO2-terminated ReO3 as well as WO3 (001) surfaces (0.170e and 0.108e, respectively). © 2022 by the authors. Licensee MDPI, Basel, Switzerland.
Ab Initio Modeling of Y and O Solute Atom Interaction in Small Clusters within the bcc Iron Lattice
(Wiley-VCH GmbH, 2018) Mastrikov, Yuri A.; Sokolov, Maksim N.; Kotomin, Eugene A.; Gopejenko, Aleksejs; Zhukovskii, Yuri F.
Small yttrium and oxygen complexes in the body‐centred cubic (bcc) iron matrix are modeled by performing density functional theory (DFT) calculations. The interaction between Y and O in isolated molecule, rock‐salt crystal is compared with that in Y/O, Y/2O, 2Y/O clusters within the bcc iron matrix. Interaction energies and electron charge redistribution are also analyzed. Among the clusters, the most stable ones are analyzed further. It is shown that chemical bonding in YO molecule and crystal is significantly stronger than in the host matrix and the main interaction in the matrix occurs with nearby Fe atoms.
Ab initio molecular dynamics simulations of negative thermal expansion in ScF3: The effect of the supercell size
(Elsevier B.V., 2020) Krack, M.; Bocharov, Dmitry; Rafalskij, Yuri; Kuzmin, Alexei; Purans, Juris
Scandium fluoride (ScF3) belongs to a class of negative thermal expansion (NTE) materials. It shows a strong lattice contraction up to about 1000 K switching to expansion at higher temperatures. Here the NTE effect in ScF3 is studied in the temperature range from 300 K to 1600 K using ab initio molecular dynamics (AIMD) simulations in the isothermal-isobaric (NpT) ensemble. The temperature dependence of the lattice constant, inter-atomic Sc–F–Sc bond angle distributions and the Sc–F and Sc–Sc radial distribution functions is obtained as a function of supercell size from 2a × 2a × 2a to 5a × 5a × 5a where a is the lattice parameter of ScF3. A comparison with the experimental Sc K-edge EXAFS data at 600 K is used to validate the accuracy of the AIMD simulations. Our results suggest that the AIMD calculations are able to reproduce qualitatively the NTE effect in ScF3, however a supercell size larger than 2a × 2a × 2a should be used to account accurately for dynamic disorder. The origin of the NTE in ScF3 is explained by the interplay between expansion and rotation of ScF6 octahedra
Ab initio simulation of (Ba,Sr)TiO3 and (Ba,Ca)TiO3 perovskite solid solutions
(Elsevier B.V., 2019) Rusevich, Leonid L.; Zvejnieks, Guntars; Kotomin, Eugene A.
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.
Ab initio simulations on charged interstitial oxygen migration in corundum
(Elsevier B.V., 2018) Platonenko, Alexander; Gryaznov, Denis; Zhukovskii, Yuri F.; Kotomin, Eugene A.
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.
Ab-Initio Calculations of Oxygen Vacancy in Ga2O3 Crystals
(Sciendo, 2021) Usseinov, A.; Koishybayeva, Zh.; Platonenko, Aleksandrs; Akilbekov, A.; Purans, Juris; Pankratov, Vladimir; Suchikova, Y.; Popov, Anatoli I.
Gallium oxide β-Ga2O3 is an important wide-band gap semiconductor. In this study, we have calculated the formation energy and transition levels of oxygen vacancies in β-Ga2O3 crystal using the B3LYP hybrid exchange-correlation functional within the LCAO-DFT approach. The obtained electronic charge redistribution in perfect Ga2O3 shows notable covalency of the Ga-O bonds. The formation of the neutral oxygen vacancy in β-Ga2O3 leads to the presence of deep donor defects with quite low concentration. This is a clear reason why oxygen vacancies can be hardly responsible for n-type conductivity in β-Ga2O3.--//-- Licenced under CC BY 4.0.
About Correctness of Graph-Based Social Network Analysis
(2017-07-24) Opmanis, Mārtiņš
Social network analysis widely uses graph techniques. Together with correct applications, in some cases, results are obtained from the graphs using paths longer than one, and due to intransitivity of relationships, several metrics and results are not applicable backward to objects in the investigated domain in a meaningful way. The author provides several examples and tries to recover roots of an incorrect application of graphs.
Abrupt elastic-to-plastic transition in pentagonal nanowires under bending
(Beilstein-Institut Zur Forderung der Chemischen Wissenschaften, 2019) Vlassov, Sergei; Mets, Magnus; Polyakov, Boris; Bian, Jianjun; Dorogin, Leonid; Zadin, Vahur
In this study, pentagonal Ag and Au nanowires (NWs) were bent in cantilever beam configuration inside a scanning electron microscope. We demonstrated an unusual, abrupt elastic-to-plastic transition, observed as a sudden change of the NW profile from smooth arc-shaped to angled knee-like during the bending in the narrow range of bending angles. In contrast to the behavior of NWs in the tensile and three-point bending tests, where extensive elastic deformation was followed by brittle fracture, in our case, after the abrupt plastic event, the NW was still far from fracture and enabled further bending without breaking. A possible explanation is that the five-fold twinned structure prevents propagation of critical defects, leading to dislocation pile up that may lead to sudden stress release, which is observed as an abrupt plastic event. Moreover, we found that if the NWs are coated with alumina, the abrupt plastic event is not observed and the NWs can withstand severe deformation in the elastic regime without fracture. The coating may possibly prevent formation of dislocations. Mechanical durability under high and inhomogeneous strain fields is an important aspect of exploiting Ag and Au NWs in applications like waveguiding or conductive networks in flexible polymer composite materials.

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