National Repository of Grey Literature 34 records found  1 - 10nextend  jump to record: Search took 0.01 seconds. 
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.
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.
Electronic structure and magnetic properties of the materials with strong electron-electron correlation
Kozub, Agnieszka Lidia ; Shick, Alexander (advisor) ; Legut, Dominik (referee) ; Minár, Ján (referee)
In this thesis, we summarize the material-specific theories of strongly correlated systems and apply them to selected materials. We describe and apply the corre- lated band theory methods: the local density approximation plus Coulomb U, and the density functional theory plus exact diagonalization of single impurity An- derson model. First, we investigate the systems containing impurity atoms: cobalt impurity located in the bulk copper and samarium, and neodymium adatoms on the surface of graphene. We present the spectral densities and study the magnetism of those compounds. Afterwards, we analyze three Np-based compounds: NpPt2In7, Np2Ni17 and NpBC. For all three compounds we analyze the spin, orbital and to- tal magnetic moments and the total density of states, as well as its projections for selected orbitals and spins. Moreover, for NpPt2In7 and NpBC we perform the to- tal energy analysis between different magnetic moment arrangements on the Np atoms.
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.
"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...
Hyperfine interactions in maghemite and magnetite particles
Křišťan, Petr
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).
New binary refractory metal-Fe intermetallic compounds for hard magnet applications
Tchaplianka, Maxim ; Shick, Alexander
We investigate theoretically the electronic and magnetic structure of Fe2Hf. The density functional theory is used to calculate the magnetic moments on individual atoms, the total and projected densities of states, and the magnetic anisotropy energy. The Fe2Hf is found to be metallic and ferrimagnetic, with the magnetic moments of Fe and Hf atoms pointing in the opposite directions. The negative magnetic anisotropy, and the “in-plane” preferential direction of the magnetization are found as a result of theoretical calculations. Our study suggests that the chemical control of the magnetic anisotropy has to be investigated in order to evaluate the potential of Fe2Hf for the permanent magnet applications.\n
Hyperfine interactions and electronic structure of magnetite
Řezníček, Richard ; Štěpánková, Helena (advisor) ; Máca, František (referee) ; Schneeweiss, Oldřich (referee)
The present work focuses on analyses of the experimental 57 Fe nuclear magnetic resonance data and the results of ab initio calculations in order to achieve deeper insight into the hyperfine and electronic structure of magnetite both below and above the Verwey phase transition. Parameters of hyperfine interactions and electronic structure data of the Cc phase of magnetite were extracted from the results of ab initio calculations based on the recently reported crystal structure. Published experimental dependences of the 57 Fe nuclear magnetic resonance frequencies on the external magnetic field direction were quantitatively reanalyzed, yielding hyperfine anisotropy data, which were compared to the analogous hyperfine field parameters from the ab initio calculations. The findings were interpreted in the context of up-to-date charge ordering models for magnetite. Attention was also paid to the impact of various cationic substitutions and defects on the electronic structure and hyperfine interactions in magnetite manifested in the 57 Fe nuclear magnetic resonance spectra: below the Verwey transition, signal intensity shifts in the spectra were analyzed; above the Verwey transition, the mean field model was adapted for an analysis of temperature dependence of satellite signal frequencies, while the anomalous...
Electronic structure and magnetic properties of the materials with strong electron-electron correlation
Kozub, Agnieszka Lidia ; Shick, Alexander (advisor) ; Legut, Dominik (referee) ; Minár, Ján (referee)
In this thesis, we summarize the material-specific theories of strongly correlated systems and apply them to selected materials. We describe and apply the corre- lated band theory methods: the local density approximation plus Coulomb U, and the density functional theory plus exact diagonalization of single impurity An- derson model. First, we investigate the systems containing impurity atoms: cobalt impurity located in the bulk copper and samarium, and neodymium adatoms on the surface of graphene. We present the spectral densities and study the magnetism of those compounds. Afterwards, we analyze three Np-based compounds: NpPt2In7, Np2Ni17 and NpBC. For all three compounds we analyze the spin, orbital and to- tal magnetic moments and the total density of states, as well as its projections for selected orbitals and spins. Moreover, for NpPt2In7 and NpBC we perform the to- tal energy analysis between different magnetic moment arrangements on the Np atoms.
Hyperfine interactions in maghemite and magnetite particles
Křišťan, Petr
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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