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Influence of cationic substitution on hyperfine interactions in magnetite
Řezníček, Richard
The subject matter of the present work is a study of a series of single crystal samples of magnetite with substitutions of zinc and titanium by means of the NMR method. Ions of the zinc substitution Zn2+ replace a part of ferric ions at tetrahedral (A) sites, while the titanium ions Ti4+ occupy octahedral (B) sites replacing iron ions Fe2.5+ . Hyperfine interactions and local electronic structure are sensitive to the presence of substitution. The case when the valence of the substitution ion is different from that of the replaced ion is of a particular interest. Resonance frequencies of nuclei in the neighbourhood of the substitution are shifted due to the modified hyperfine field, thus satellite lines can be observed in NMR spectra. Temperature dependences of spectra above the Verwey transition were measured in a zero external magnetic field. Additionally, NMR spectra were also acquired at the temperature of 4.2 K. Temperature dependences of frequencies of main lines and satellite signals in the spectra above the Verwey transition were constructed and compared to the data for pure magnetite and magnetite with other substitutions and with cationic vacancies. Furthermore, variations of widths of A lines against the temperature above the Verwey transition were found and discussed.
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Chemical reactivity of metal-supported ceria thin films: a density functional study
Szabová, Lucie ; Matolín, Vladimír (advisor) ; Balducci, Gabriele (referee) ; Chvoj, Zdeněk (referee)
Title: Chemical reactivity of metal-supported ceria thin films: a density func- tional study Author: Lucie Szabová Department: Department of Surface and Plasma Science Supervisor: Prof. RNDr. Vladimír Matolín, DrSc., Department of Surface and Plasma Science, FMF, CU Abstract: The present work is a theoretical analysis based on numerical DFT+U simulations investigating the physical and chemical properties of ultrathin ceria films supported by Cu(111). Such materials exhibit high activity towards several important reactions in heterogeneous catalysis such as water-gas shift and CO oxidation, with important applications also for renewable energy technologies such as fuel cells. We provide evidence of the influence of film thickness on the electronic and structural properties as well as on the reactivity of ultrathin ceria films supported by copper. The calculations combined with scanning tunneling microscopy experiments show that one monolayer thin film of ceria on Cu(111) is charged, strained and contains oxygen vacancies due to the limited thickness of the film. The influence of the film thickness on the reactivity of thin ceria films was explored for the case of water adsorption and dissociation. Significant differences were shown for water adsorption and dissociation on one-monolayer ceria compared to thicker films,...
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"Ab initio" study of Cu-Ce-O interface
Szabová, Lucie ; Matolín, Vladimír (advisor) ; Chvoj, Zdeněk (referee)
"Ab initio" study of interface Cu-Ce-O Abstract: The present work is a theoretical analysis based on the numerical DFT+U simulations investigating the structural and electronic properties of Cu/CeO2 model systems, which have important applications as heterogeneous catalysts for environment protection and energy sources. We provide a detailed insight into the cohesion of the interface between metal Cu nanoparticles supported on CeO2 substrates. This issue is analyzed both in context of small supported Cu clusters as well as for the extended interface underneath Cu nanoparticles on ceria surfaces. These cases were modelled with a Cu(111)/CeO2(111) interface and with a Cu adatom adsorbed at the oxidized and reduced CeO2(111) surface, respectively. The thesis provides a direct correlation between the cohesive and electronic properties mediated by the charge transfer process. The reduction of surface cerium atoms in the presence of copper either in form of adatom, thin copper layer or a slab of copper is predicted to result from charge transferred from the metal. Since cerium reduction is suggested to play an important role into the catalytic activity of ceria-based catalysts, by predicting the reduction of cerium ions in the presence of copper it can be expected that Cu/CeO2 systems will have important...
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Hyperfine interactions in maghemite and magnetite particles
Křišťan, Petr ; Štěpánková, Helena (advisor) ; Procházka, Ivan (referee)
Thesis is aimed at studying of magnetic iron oxide particles of submicron and nanoscale dimensions by means of nuclear magnetic resonance (NMR). 57 Fe NMR inves- tigations were carried out in composite bentonite/maghemite with respect to tempera- ture of calcination (Tcalc) during the sample preparation and in magnetite submicron powders with respect to various range of the particles size. One of the main findings is that increasing Tcalc improves resolution in the NMR spectra, which is most likely connected with higher degree of atomic ordering in the spinel structure. Evaluating the integral intensities of NMR spectra allowed us to determine the relative content of maghemite phase in particular samples of the series: the content rapidly grows for Tcalc up to ∼420 deg. An approach to distinguish signal from tetrahedral and octahedral irons was developed and tested on pure maghemite sample. Analysis based on vacancy- distribution models was performed in the spinel structure and the results were compared to the experiment. 57 Fe NMR spectra in submicron magnetite samples were found to differ markedly from spectrum of a single crystal. It was concluded that the investigated powders possess high amount of defects in the crystal structure or contain additional phase (probably closely related to the maghemite phase).
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STRAIN ENGINEERING OF THE ELECTRONIC STRUCTURE OF 2D MATERIALS
del Corro, Elena ; Peňa-Alvarez, M. ; Morales-García, A. ; Bouša, Milan ; Řáhová, Jaroslava ; Kavan, Ladislav ; Kalbáč, Martin ; Frank, Otakar
The research on graphene has attracted much attention since its first successful preparation in 2004. It possesses many unique properties, such as an extreme stiffness and strength, high electron mobility, ballistic transport even at room temperature, superior thermal conductivity and many others. The affection for graphene was followed swiftly by a keen interest in other two dimensional materials like transition metal dichalcogenides. As has been predicted and in part proven experimentally, the electronic properties of these materials can be modified by various means. The most common ones include covalent or non-covalent chemistry, electrochemical, gate or atomic doping, or quantum confinement. None of these methods has proven universal enough in terms of the devices' characteristics or scalability. However, another approach is known mechanical strain/stress, but experiments in that direction are scarce, in spite of their high promises.\nThe primary challenge consists in the understanding of the mechanical properties of 2D materials and in the ability to quantify the lattice deformation. Several techniques can be then used to apply strain to the specimens and thus to induce changes in their electronic structure. We will review their basic concepts and some of the examples so far documented experimentally and/or theoretically.
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Ab-initio calculation of structures´ stability of Ni-N compounds
Šárfy, Pavlína ; Vřešťál,, Jan (referee) ; Šob,, Mojmír (advisor)
The present thesis is devoted to ab initio study of electronic structure of nickel nitrides NiN, Ni2N, Ni3N and Ni4N. The results are used to predict the most stable structures for each composition. The total energies and the electronic structures are calculated by means of the pseudopotential method implemented in the Abinit code and by full-potential linearized augmented plane wave (FLAPW) method incorporated in the Wien2K code. For the exchange-correlation energy, both the local density approximation (LDA) and generalized approximation (GGA) are employed. We predicted the face centered cubic structure B3 as the most stable modification of NiN, the primitive tetragonal structure C4 as the most stable modification of Ni2N, the hexagonal structure as the most stable modification of Ni3N (in agreement with experimental data) and the primitive cubic structure as the most stable modification of Ni4N.
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Stability of iron, cobalt and nickel carbides and nitrides from first principles
Svatoň, Josef ; Vřešťál, Jan (referee) ; Šob, Mojmír (advisor)
The present hesis is devoted to crystal structure stability of cobalt and nickel carbides and nitrides and exactly structures NiC and CoN. In this case we understand this structure as lowest energetic status which the crystals are in. Using computational program ABINIT we get numerical solutions of electronic structers to predicate staility of chosen structures. All structures are compared with experimental observation. The total energies and the electronic structures are calculated by means of pseudopotencial method implemented in ABINIT. As a solution of my observation consider I the structures of the zincblende for both solids NiC and CoN, thus face centered cubic structure.
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Qantum-mechanical study of structural stability of Ni4N allotropes
Hemzalová, P. ; Friák, Martin ; Šob, Mojmír ; Neugebauer, J.
Parameter-free density functional theory (DFT) calculations of Ni4N in eight crystallographic phases were performed using the pseudopotential approach implemented in the VASP code; the exchange-correlation energy was evaluated within the generalized gradient approximation (GGA). In agreement with experiments, the cubic structure with Pearson symbol cP5, space group Pm-3m (221), has been found to be the most stable. It is also the only thermodynamically stable structure at T=0 K with respect to decomposition into elemental Ni crystal and N2 gas phase. We determine structural, thermodynamic, electronic, magnetic and elastic properties of all eight Ni4N allotropes studied. The thermodynamic stability and bulk modulus is found to be anti-correlated. For the cubic allotropes, we predict a complete set of single-crystalline elastic constants, directional dependence of the single-crystalline Young modulus and homogenized polycrystalline elastic moduli.
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Investigating ground state of nickel nitrides NiN and Ni2N with the help of quantum-mechanical calculations
Elstnerová, P. ; Friák, Martin ; Šob, Mojmír ; Neugebauer, J.
We have employed quantum mechanical calculations to identify ground-state structures of nickel nitrides NiN and Ni2N for which experimental data are lacking. In total 19 crystalline phases have been calculated for which not only thermodynamic but also structural and selected elastic properties have been determined. Employing density functional theory (DFT) methods, the total energies were calculated by means of a pseudopotential approach implemented in the VASP code and selected states were benchmarked by the full-potential linearized augmented plane wave (FP-LAPW) method implemented in the WIEN2k code. For the exchange-correlation energy the generalized gradient approximation (GGA) has been used.
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