A. Kiejna

University of Wroclaw, Vrotslav, Lower Silesian Voivodeship, Poland

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Publications (88)207.32 Total impact

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    ABSTRACT: This is the first of two papers dealing with the adsorption of Au and formation of Aunnanostructures (n = 1–4) on hematite (0001) surface and adsorption of CO thereon. The stoichiometric Fe-terminated (0001) surface of hematite was investigated using density functional theory in the generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) form with Hubbard correction U, accounting for strong electron correlations (PBE+U). The structural, energetic, and electronic properties of the systems studied were examined for vertical and flattened configurations of Aunnanostructures adsorbed on the hematitesurfaces. The flattened ones, which can be viewed as bilayer-like structures, were found energetically more favored than vertical ones. For both classes of structures the adsorption binding energy increases with the number of Au atoms in a structure. The adsorption of Aun induces charge rearrangement at the Aun/oxide contact which is reflected in work function changes. In most considered cases Aunadsorption increases the work function. A detailed analysis of the bonding electron charge is presented and the corresponding electron charge rearrangements at the contacts were quantified by a Bader charge analyses. The interaction of a CO molecule with the Aunnanostructures supported on α-Fe2O3 (0001) and the oxide support was studied. It is found that the CO adsorption binding to the hematite supported Aun structures is more than twice as strong as to the bare hematitesurface. Analysis of the Bader charges on the atoms showed that in each case CO binds to the most positively charged (cationic) atom of the Aun structure. Changes in the electronic structure of the Aun species and of the oxide support, and their consequences for the interactions with CO, are discussed.
    No preview · Article · Jan 2016 · The Journal of Chemical Physics
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    ABSTRACT: Adsorption of oxygen on the (110) surface of epitaxial iron films on tungsten (110) was studied using low-energy electron diffraction (LEED), low-energy electron microscopy (LEEM) and Auger electron spectroscopy (AES) within an exposure range of 0–300 Langmuir (L). Selected oxygen adsorption structures on Fe(110) reported in the literature were critically compared and revised in reference to the present study. The initial adsorption of ¼ oxygen monolayer (ML) resulting in the commonly observed (2×2) structure was followed by a structure that was frequently termed as (3×1). Its complex LEED pattern was ultimately resolved and interpreted as originating from two structural domains of a large oblique unit cell (eight times larger than the substrate unit cell) and ⅜ oxygen coverage. A new (3×2) structure was identified at a coverage of ⅔. The domain interpretation of last two structures was verified by LEEM and confirmed by density functional theory calculations. The onset of oxygen-iron bonding formation was recognized by the change in the symmetry of the LEED pattern and the shape of the iron AES signal. Finally, the formation of an iron oxide FeO(111) monolayer was evidenced at the oxygen exposure of approximately 300 L.
    No preview · Article · Jan 2016 · The Journal of Physical Chemistry C
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    Damian Wiśnios · Adam Kiejna · Józef Korecki
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    ABSTRACT: The adsorption of MgO molecules on a Fe(001) surface was studied using density functional theory (DFT) and projector augmented wave methods. The energetically most favored configurations for different adsorption sites considered were identified. The most preferable adsorption geometry is when the MgO molecules are parallel to the surface, with Mg in the interstitial site and O in on-top of the Fe atom. During the adsorption of subsequent MgO molecules in this geometry, a sharp, non-oxidized interface is formed between the MgO adlayer and Fe(001) surface. The adsorption of MgO perpendicular to the surface, with oxygen incorporated in the topmost Fe layer is less probable, but may lead to the formation of the FeO layer when stabilized with an excess of oxygen atoms. Structural, electronic and magnetic properties of both interface types were examined for the MgO coverage from 1/9 to 1 monolayer (ML). Electronic and magnetic properties are sensitive to the MgO coverage. For lower coverage of MgO, clear hybridization between the Fe 3d and O 2p states is shown. The average magnetic moment of the surface Fe atoms is reduced with coverage, achieving 2.78 $\mu_{\rm B}$ for 1 ML of MgO.
    Preview · Article · Jul 2015 · Physical Review B
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    ABSTRACT: Despite extensive studies of transition metal (TM) clusters supported on ceria (CeO2 ), fundamental issue as the role of the TM atoms in the change of the oxidation state of Ce atoms is still not well understood. In this work, we will report a theoretical investigation based on static and ab-initio molecular dynamics density functional theory calculations of the interaction of 13-atom clusters (Pd, Ag, Pt, Au) with the unreduced CeO2 (111) surface represented by a large surface unit cell and employing Hubbard corrections for the strong on-site Coulomb correlation in the Ce f -electrons. We found that the TM13 clusters form pyramidal-like structures on CeO2 (111) in the lowest energy configurations with the following stacking sequence, TM/TM4 /TM8 /CeO2 (111), while TM13 adopts two-dimensional structures at high energy structures. TM13 induces a change in the oxidation state of few Ce atoms (3 of 16) located in the topmost Ce layer from CeIV (itinerant Ce f -states) to CeIII (localized Ce f -states). There is a charge flow from the TM atoms to the CeO2(111) surface, which can be explained by the electronegativity diffence between the (Pd, Ag, Pt, Au) atoms and O, however, the charge is not uniformely distributed on the topmost O layer due to the pressure induced by the 13-atom clusters on the electron density, which yields a decreasing in the ionic charge of the O ions located below the cluster and an increasing in the remaining O ions. Due to the charge flow mainly from the TM8-layer to the topmost O-layer, the charge cannot flow from the CeIV atoms to the O atoms with the same magnitude as in the clean CeO2 (111) surface. Consequently, the effective cationic charge decreases mainly for the Ce atoms that have a bond with the O atoms not located below the cluster, and hence, those Ce atoms change their oxidation state from IV to III. This increases the size of the CeIII compared with CeIV cations, which builds-in a strain within the topmost Ce layer, and hence, it contributes also to affect the location of the CeIII cations and the structure of the TM13 clusters.
    Preview · Article · Apr 2015 · Physical Chemistry Chemical Physics
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    Tomasz Ossowski · Adam Kiejna
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    ABSTRACT: We performed density functional theory calculations of an oxygen adsorption on a Fe(110) surface to examine structural, electronic, and magnetic properties of O/Fe(110) systems. Oxygen adsorption in different on-surface sites was considered for O coverages varying between a 1/4 and 1 monolayer (ML). It has been found that an oxygen adsorption in long bridge sites is less favorable than in pseudo threefold hollow sites, although at low coverage (1/4 ML) the adsorption in both sites is nearly degenerated in energy. At higher coverages, oxygen atoms markedly prefer to bind in pseudo threefold coordinated hollows. This is in contrast to the experiments which suggested that the long bridge sites are most stable, and to earlier theoretical studies which reported the threefold coordinated hollow sites to be unstable. We show that the preference for an oxygen ad-sorption in the threefold hollow sites which results from calculations presented in this work can be reconciled with experimental observations of low-coverage adsorption of O in long bridge sites. Consequences of this change in the order of the stability of on-surface sites for the O atoms adsorption in subsurface sites at the O-precovered Fe(110) surface are briefly discussed.
    Full-text · Article · Mar 2015 · Surface Science
  • Tomasz Pabisiak · Adam Kiejna
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    ABSTRACT: The structure and electronic properties of different terminations of hematite (0001) and magnetite (111) surfaces upon submonolayer Fe adsorption were studied using the spin-polarized density functional theory (DFT) including the Hubbard correction term U (DFT+U). On both oxides the Fe atoms were adsorbed on the most stable iron and oxygen terminated surfaces. The results show that Fe atoms bind strongly both to hematite and magnetite surfaces, however, the binding is distinctly stronger at the oxygen than at the iron terminated surfaces. For both oxides and surface terminations the binding energy of the Fe decreases with increasing coverage, which indicates substantial repulsive interactions between Fe adatoms. On the hematite surface, the most stable sites for Fe adsorption are bulk continuation sites which result in formation of the Fe-rich terminations. On the magnetite surface, the bulk continuation site is favored only for Fe adsorption on the oxygen terminated surface while on the iron terminated one Fe adsorbs in a position closer to the surface iron layer. Submonolayer coverages of Fe modify substantially the surface electronic structure of the oxides and, depending on the termination, can change its character from half-metallic to insulating one, and vice versa.
    No preview · Article · Oct 2014 · The Journal of Chemical Physics
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    ABSTRACT: Several experimental and theoretical studies have suggested that the formation of surface alloys or the deposition of strained transition metal (TM) monolayers (ML) on TM supports can be considered as a route for the designing of new catalysts. In this work, we report an extensive first-principles investigation based on density functional theory of the adsorption of TM (Rh, Pd, Ir, Pt) on the Cu(111) and Au(111) surfaces considering TM coverages ranging from 1/9, 2/9, up to 1 ML. Although there are clear differences in the atomic radii of the Cu, Rh, Pd, Ir, Pt, and Au atoms, at low TM coverages, both systems exhibit similar behavior, namely, the lowest energy adsorption site for a single TM adatom is not in the hollow sites on the surface, but in the lattice sites located in the topmost layer. For TM/Au(111), this trend follows adatom by adatom up to the limit in which all the substrate Au atoms are exposed to the vacuum region, with the underlying TM adatoms, and it is also valid for Rh/Cu(111). For Pd, Ir, and Pt on Cu(111), the same trend is observed up to 4/9, 8/9, and 6/9 TM coverages, and the adatoms are exposed to the vacuum region for higher coverages. For TM/Au(111), our analyses indicate a tensile strain built-in due to the mixture of adatoms with smaller radii with Au with a larger radius in the same first (topmost) surface layer, while a compressive strain can be seen for TM/Cu(111), in particular for Pd, Ir, and Pt at high coverages, which favors the location of the TM adatoms on Cu(111). Judging by the Pauling electronegativity scale, we would expect a different behavior for the substrate work function change upon TM adsorption on Cu(111) and Au(111). However, a similar behavior was obtained for the lowest energy configurations on both substrates. This is rationalized in terms of the electronegativity differences, geometrical effect of atomic smoothing, the insertion of the adatoms in the first (topmost) surface layer, and the exposed layer to the vacuum region.
    No preview · Article · Aug 2014 · The Journal of Physical Chemistry C
  • Adam Kiejna · Tomasz Pabisiak
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    ABSTRACT: The detailed structure of hematite (0001) surfaces is both of fundamental interest and of crucial meaning in understanding the reactivity of the surfaces with respect to different adsorbates. The structure and electronic properties of mixed terminations of the α-Fe2O3(0001) surface were studied with the spin-polarized density functional theory (DFT) and the DFT+U methods in order to explore possibilities of their stable coexistence. The DFT+U results show that the mixed terminated slabs consisting of Fe- and O-terminated domains of large periodicity are energetically more stable than those resulting from the combination of pure Fe- and O-terminated fragments.
    No preview · Article · Nov 2013 · The Journal of Physical Chemistry C
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    E. Wachowicz · A. Kiejna
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    ABSTRACT: The constant adsorption energy surfaces for hydrogen adsorbed on Si- and C-terminated hexagonal 4H-SiC{0001} surfaces have been calculated within density functional theory framework. The two unreconstructed and one reconstructed √3 × √3 surfaces were taken into account. We show that on all surfaces there is a global energy minimum indicating the most favourable adsorption site corresponding to H atom adsorption on-top of the topmost substrate layer atom. In case of reconstructed surface, there is another small and shallow local minimum. Moreover, the diffusion barrier is much higher at reconstructed surface than at unreconstructed ones.
    Preview · Article · Nov 2013 · Acta Physica Polonica Series a
  • Elwira Wachowicz · Adam Kiejna
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    ABSTRACT: The changes in the atomic and electronic structure of Si- and C-terminated hexagonal SiC{0001} surfaces resulting from on-surface and subsurface hydrogen adsorption have been studied within the density functional theory framework. Hydrogen coverages ranging from a submonolayer to one monolayer were considered. Our results show that a monolayer of adsorbed H almost completely suppresses the relaxation of the SiC surface atomic layers. On both terminations H binds strongly to the surface and the binding is about 2 eV stronger in on-surface sites than subsurface. Hydrogen binding to the C-terminated surface varies very little with coverage and is distinctly stronger than to the Si-terminated surface.
    No preview · Article · Sep 2012 · Journal of Physics Condensed Matter
  • Duy Le · Maral Aminpour · Adam Kiejna · Talat S Rahman
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    ABSTRACT: We present the results of ab initio electronic structure calculations for the adsorption characteristics of three amine molecules on Au(111), which show that the inclusion of van der Waals interactions between the isolated molecule and the surface leads in general to good agreement with experimental data on the binding energies. Each molecule, however, adsorbs with a small tilt angle (between -5 and 9°). For the specific case of 1,4-diaminobenzene (BDA) our calculations reproduce the larger tilt angle (close to 24°) measured by photoemission experiments, when intermolecular (van der Waals) interactions (for about 8% coverage) are included. These results point not only to the important contribution of van der Waals interactions to molecule-surface binding energy, but also that of intermolecular interactions, often considered secondary to that between the molecule and the surface, in determining the adsorption geometry and pattern formation.
    No preview · Article · Apr 2012 · Journal of Physics Condensed Matter
  • Adam Kiejna · Tomasz Pabisiak
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    ABSTRACT: The structure and electronic properties of different terminations of the α-Fe(2)O(3)(0001) surface were studied with spin-polarized density functional theory (DFT) and the DFT + U method accounting for the effect of strong on-site Coulomb correlations. The results for lattice relaxation, electronic and magnetic properties are presented and discussed. Though the DFT and DFT + U methods provide qualitatively very similar surface geometries they differ very much in the prediction of the electronic and magnetic properties, and the surface energetics of the clean α-Fe(2)O(3)(0001). The most stable single iron terminated (0001) surface and the oxygen-rich termination were chosen to study Au and Pd atom adsorption. The results show that both Au and Pd bind strongly to hematite surfaces and induce large changes in their geometry. The DFT + U bonding is weaker by 0.3-0.6 eV than DFT on the iron terminated surface and about 2 eV stronger on the oxygen terminated one. The binding is stronger for Pd than Au and for both adsorbates is distinctly stronger at the oxygen than at the iron terminated surface. On the iron terminated surface the adsorption binding energy per adatom increases both with Au and Pd coverage, whereas for the oxygen terminated one the opposite trend is observed.
    No preview · Article · Mar 2012 · Journal of Physics Condensed Matter
  • Adam Kiejna · Tomasz Pabisiak · Tomasz Ossowski
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    ABSTRACT: Ultra-thin films and nanostructures formed by noble metals on oxide surfaces exhibit enhanced catalytic activity for CO oxidation. We used the spin-polarized density functional theory (DFT) and the DFT+U method, accounting for the strong on-site Coulomb correlations, to study the submonolayer adsorption of Au/Pd atoms on two stable iron-oxide surfaces: hematite (0001) and a magnetite (111). For each surface, adsorption on two terminations has been studied: one terminated with iron and the other with oxygen. Both Au and Pd bind strongly to hematite and magnetite surfaces and induce large changes in their geometry. DFT and DFT+U provide qualitatively similar surface geometries but they differ much in the prediction of the surface energetics and the electronic and magnetic properties of the oxides. Pd binds stronger than Au both to hematite and magnetite surfaces and the Au/Pd bonding to the O-terminated surface is distinctly stronger than that to the Fe-terminated one. For hematite, the DFT+U bonding is by 0.3-0.6 eV weaker than DFT on the Fe-terminated surface and about 2 eV stronger on the O-terminated one. For magnetite, in each case, DFT+U gives stronger bonding than DFT. The differences between DFT and DFT+U results are discussed based on the calculated electronic structure.
    No preview · Article · Feb 2012
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    Adam Kiejna · Tomasz Ossowski · Tomasz Pabisiak
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    ABSTRACT: The spin-density functional theory (DFT) and DFT+$U$ with Hubbard $U$ term accounting for on-site Coulomb interactions were applied to investigate structure, stability, and electronic properties of different terminations of the Fe$_3$O$_4$(111) surface. All terminations of the ferrimagnetic Fe$_3$O$_4$(111) surface exhibit very large (up to 90%) relaxations of the first four interlayer distances, decreasing with the oxide layer depth. Our calculations predict the iron terminated surface to be most stable in a wide range of the accessible values of the oxygen chemical potential. The adsorption of Au and Pd on two stable Fe- and O-terminated surfaces is studied. Our results show that Pd binds stronger than Au both to the Fe- and O-terminated surface. DFT+$U$ gives stronger bonding than DFT. The bonding of both adsorbates to the O-terminated magnetite surface is by 1.5-2.5 eV stronger than to the Fe-terminated surface.
    Full-text · Article · Dec 2011 · Physical review. B, Condensed matter
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    ABSTRACT: Radiation damage may modify the segregation state and phase separation conditions in Fe–Cr alloys with compositions and temperatures of technological interest. We use Metropolis Monte Carlo simulations to study segregation effects at the best stable variant of the Σ = 5 (2 1 0) grain boundary, in the 5–10 at.% range of Cr composition. The role of irradiation induced vacancies in segregation is discussed. At a low Cr composition, an oscillatory Cr segregation profile is evidenced in the vicinity of the boundary. Under specific conditions, Cr ordering is observed close to the boundary. The correlations between such ordering and local stress is discussed. The binding energy of vacancies to specific sites of the grain boundary is found to be positive and the interplay between Cr segregation and the clustering of vacancies at the boundary is discussed.
    No preview · Article · Jul 2011 · Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms
  • T. Pabisiak · A. Kiejna
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    ABSTRACT: The adsorption of gold atoms and formation of nanostructures on the rutile TiO2(110) surface with different degree of oxygen reduction was studied from first principles. The Au atoms adsorb strongest at oxygen vacancy sites. Starting from a very low coverage limit the potential energy profiles or diffusion paths of the adsorbed Au monomers and dimers were calculated. Stable structures of two to nine Au atoms arranged in finite and infinite rows and in the shape of finite-size clusters were determined. All these structures are found to bind to the reduced surface stronger than 2 eV/atom. The elongated Au row-like structures bind by about 0.1 eV stronger than 3D clusters, suggesting a preference for the 1D-like Au growth mode on the missing-row reconstructed TiO2(110).Research highlights► Formation of Au nanostructures on the rutile TiO2(110) with different degree of O reduction. ► Au atoms adsorb strongest at oxygen vacancy sites. ► Potential energy profiles of the adsorbed Au monomers and dimers. ► Stable structures of different shape of two to nine Au atoms were determined. ► Infinitely long Au rows most stable of all investigated structures on the reconstructed TiO2(110).
    No preview · Article · Apr 2011 · Surface Science
  • E Wachowicz · A Kiejna
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    ABSTRACT: The effects of several metalloid (B), metalloid-like (C, P) and nonmetallic impurities (N, O and S) on structure, energetics and mechanical properties of Σ3(1 1 1) and Σ5(2 1 0) grain boundaries (GBs) in ferromagnetic α-Fe have been studied from first principles. For two different concentrations and positions of impurity atoms, the variations in GB properties have been analysed. Most of the impurities enhance the relaxation of the interplanar spacing of the pure grains. Interstitial impurities at both GBs are shown to increase the separation of the grains while substitutional ones in general either do not alter or decrease the grains' separation. It is shown that at the Σ5 GB for all impurity atoms considered the positions in the boundary layer are energetically favoured independently of the interstitial or substitutional site, whereas the enrichment of the Σ3 GB is favoured for the impurities of the interstitial sites as well as for a substitutional P and C. We have found that, in most cases, impurity atoms both in interstitial and substitutional positions at GBs act as embrittlers. Interstitial B, P and C strengthen the Σ3 GB cohesion, and B and C at lower concentrations strengthen the Σ5 GB. All substitutional impurities, except for B, weaken the GBs. The magnetic moments on the impurities are very small and in most cases align antiparallel to the moments on the neighbouring Fe atoms.
    No preview · Article · Jan 2011 · Modelling and Simulation in Materials Science and Engineering
  • T Ossowski · E Wachowicz · A Kiejna
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    ABSTRACT: The structural, cohesive, and magnetic properties of (111) and (210) tilt grain boundaries (GBs) in pure Cr, and in Cr with Fe additions, are studied from first principles. Different concentration and position of solute atoms are considered. Our calculations show that Fe atoms placed in the GB interstice enhance cohesion, whereas Fe substituted for one of the Cr layer atoms, in most cases, has very small (or negligible) effect on the cohesion at GBs in Cr. We have found that Fe additions show a tendency to enrich the boundaries in Cr. In the presence of Fe additions the magnetic moments on the GB host atoms are substantially modified and those on Fe impurities are reduced. In most cases the moments on Fe additions remain much higher than the local moments on the Cr atoms.
    No preview · Article · Dec 2009 · Journal of Physics Condensed Matter
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    E. Wachowicz · T. Ossowski · A. Kiejna
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    ABSTRACT: Structural, cohesive, and magnetic properties of two symmetric $\Sigma3(111)$ and $\Sigma5(210)$ tilt grain boundaries (GBs) in pure bcc Fe and in dilute FeCr alloys are studied from first principles. Different concentration and position of Cr solute atoms are considered. We found that Cr atoms placed in the GB interstice enhance the cohesion by 0.5-1.2 J/m$^2$. Substitutional Cr in the layers adjacent to the boundary shows anisotropic effect on the GB cohesion: it is neutral when placed in the (111) oriented Fe grains, and enhances cohesion (by 0.5 J/m$^2$) when substituted in the boundary layer of the (210) grains. The strengthening effect of the Cr solute is dominated by the chemical component of the adhesive binding energy. Our calculations show that unlike the free iron surfaces, Cr impurities segregate to the boundaries of the Fe grains. The magnetic moments on GB atoms are substantially changed and their variation correlates with the corresponding relaxation pattern of the GB planes. The moments on Cr additions are 2-4 times enhanced in comparison with that in a Cr crystal and are antiparallel to the moments on the Fe atoms.
    Full-text · Article · Mar 2009 · Physical review. B, Condensed matter
  • E. Wachowicz · A. Kiejna
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    ABSTRACT: The effect of B, N and O impurities placed in interstitial and substitutional positions at symmetric Σ5(210) Fe grain boundary is studied by means of first principles calculations. Full relaxation of supercell shape and volume is applied which results in stable asymmetric grain boundaries, depending on the impurity and its position. In all cases the big shifts (0.47–2.33Å) of grains with respect to each other are observed. The equilibrium distance between the grains is decreased for impurities in substitutional positions, and increased for atoms in interstitial sites, compared to the relaxed clean GB. We found that nitrogen both in interstitial and substitutional positions, and boron in substitutional position enhance cohesion while oxygen in both positions, and interstitial boron, are embrittlers. The magnetic moments of Fe atoms at clean grain boundary are remarkably increased compared to the bulk. They tend to the bulk value in the middle of the grain in an oscillatory way.
    No preview · Article · Oct 2008 · Computational Materials Science

Publication Stats

1k Citations
207.32 Total Impact Points

Institutions

  • 1981-2015
    • University of Wroclaw
      Vrotslav, Lower Silesian Voivodeship, Poland
  • 2012
    • University of Warsaw
      • Interdisciplinary Centre for Mathematical and Computational Modelling
      Warszawa, Masovian Voivodeship, Poland
  • 1981-2011
    • Wyższa Szkoła Handlowa we Wrocławiu
      Vrotslav, Lower Silesian Voivodeship, Poland
  • 2007-2008
    • University of Vienna
      • Fakultät für Physik
      Wien, Vienna, Austria
  • 2001
    • Chalmers University of Technology
      • Department of Applied Physics
      Goeteborg, Västra Götaland, Sweden