<p>A parameter free approach for the calculation of the crystal field splitting of the lowest Russel-Saunders <i>J</i>-multiplet in <i>f</i>-electron systems has been developed and applied to selected compounds. The developed theory is applicable to general symmetries and is based on symmetry constrained density functional theory calculations in the local density or in the generalised gradient approximation.</p><p>The magnetocrystalline anisotropy of Gd has been analysed. It has been shown that the peculiar orientation of the easy axis of magnetisation is consistent with an <i>S</i>-ground state. Further, the temperature dependence of the easy axis of magnetisation has been investigated and it has been shown that the temperature driven reduction of the effective magnetisation is the principal mechanism responsible for it.</p><p>A new method has been developed that allows for theoretical studies of the electronic structure and total energy of elements and compounds in an intermediate valence regime. The method combines model and first-principles band structure calculations, therefore being accurate and computationally efficient. It has been applied to Yb metal under pressure obtaining a remarkable agreement with experimental observations for the equation of state and the x-ray absorption spectroscopy.</p>

The main purpose of this thesis is to theoretically study energy levels and intra-electronic transition intensities for various f-electron systems. The f-f electronic dipole transitions are parity-forbidden for a free ion but become non-zero when the ion is subject to a crystal-field. This is commonly described within the framework of Judd-Ofelt theory which accounts for the mixing of odd parity into the wave-functions. Some refinements and quantitative studies have been made by applying many-body perturbation theory, or the perturbed functions approach, to obtain effective dipole operators due to correlation, spin-orbit and higher order crystal-field effects not included in Judd-Ofelt theory. A software for the computation of f-electron multiplets and Stark levels was implemented and published as well. The single- and pair-functions used for the evaluation of intensity parameters were obtained by solving various inhomogeneous Schrödinger equations. The wave-functions and energies obtained by diagonalizing an effective Hamiltonian have been used together with the oscillator strength methods to simulate absorption spectrum. Consistent crystal-field parameters applied in some of the papers were obtained by fitting crystal polarizabilities to reflect the experimental Stark levels. The same crystal model was then used to generate odd crystal field parameters needed for the f-f transition intensities. The total effect of these refinements are spectral features that usually agree well with experimental findings. Some of these methods have also been applied and seen to be quite useful for the understanding of optical fiber amplifiers frequently used in today's optical networks. Finally, a finite-difference approach was applied for the Helium iso-electronic sequence. The exact wave-function was expanded in a sum of partial waves, and accurate ground- and excited state energies were obtained by using the iterative Arnoldi approach.

A parameter free approach for the calculation of the crystal field splitting of the lowest Russel-Saunders J-multiplet in f-electron systems has been developed and applied to selected compounds. The developed theory is applicable to general symmetries and is based on symmetry constrained density functional theory calculations in the local density or in the generalised gradient approximation. The magnetocrystalline anisotropy of Gd has been analysed. It has been shown that the peculiar orientation of the easy axis of magnetisation is consistent with an S-ground state. Further, the temperature dependence of the easy axis of magnetisation has been investigated and it has been shown that the temperature driven reduction of the effective magnetisation is the principal mechanism responsible for it. A new method has been developed that allows for theoretical studies of the electronic structure and total energy of elements and compounds in an intermediate valence regime. The method combines model and first-principles band structure calculations, therefore being accurate and computationally efficient. It has been applied to Yb metal under pressure obtaining a remarkable agreement with experimental observations for the equation of state and the x-ray absorption spectroscopy.

The linewidths and line positions of 4I15/2 to 4I13/2 absorption transitions of trivalent erbium in CsCdBr₃ and CaF₂ were measured as a function of temperature from approximately 10 K to 300 K. By comparing the temperature dependence of these transitions with theoretical models of electron-phonon coupling the primary mechanism involved were determined. For Er3+ doped CaF₂ the Raman scattering processes dominated the line broadening, although the single-phonon direct processes were significant enough to drastically alter the values of the Raman scattering electron-phonon coupling constant. In comparison, the Er³⁺ doped CsCdBr₃ had negligible Raman scattering contributing to the line broadening. This is likely due to the exceedingly low Debye temperature and consequently low number of phonons, reducing the likelihood of two-phonon processes relative to single-phonon processes. The results were then analysed in terms of the bond length with the ligands and compared with other studies showing that as the bond length gets shorter the electron-phonon coupling constant associated with Raman scattering is expected to get smaller. To explain the line shifts in CaF₂:Er³⁺ it is necessary to treat the Debye temperature as a parameter. This indicates that the phonon modes causing the line shifts are different from those causing the line broadening, and in most of the cases are vibronic processes rather than Raman processes. In the case of CsCdBr₃:Er³⁺ there is very little difference between the fits using the fixed Debye temperature and a varied Debye temperature in over half of the cases examined. Given the distribution of phonon modes, it is likely that this small difference arises because combinations of these processes are of similar intensity, meaning that a combination of Raman and optical phonon modes are likely causing the line shift.

Thesis (M.S.)--Ohio University, August, 1997. / Title from PDF t.p.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O objetivo deste trabalho foi investigar amostras do tipo MgSiO dopadas com 1,0% e 5,0% de Mn2+, obtidas em temperatura ambiente que luminesceram na faixa do vermelho. As amostras foram produzidas por reação de estado sólido sob alta temperatura e caracterizadas e pelo método de difração de raios X. Para análise das características físicas presentes na amostra após a dopagem, usamos a espectroscopia de fotoluminescência. Através da teoria do campo cristalino, dos espectros de luminescência e excitação e com a ajuda do diagrama e das matrizes de Tanabe-Sugano, foi possível identificar as luminescências, os níveis de energia excitados, os parâmetros de energia do campo cristalino, os parâmetros de Racah e a simetria do sítio ocupado por Mn2+. / The aim this work was investigate ceramic samples of type MgSiO doped with 1,0% and 5,0% of Mn2+, obtained with room temperature. The samples were produced by solid-state reaction at high temperature and characterized by the method X ray Diffraction. For analysis of the physics characteristics in the sample after doping, we use spectroscopy photoluminescence. Through theory crystal field, luminescence and excitation spectra, and with help of diagram and Tanabe-Sugano matrices, could be identified excited energy levels, the parameters of energy crystal field, the parameters of Racah and the symmetry of the site occupied by Mn2+.

Parmi l’infinité de compositions verrières, les silicates représentent 90 % de la production mondiale de verre et sont utilisés pour de nombreuses applications industrielles (par exemple l’automobile, le bâtiment ou les panneaux solaires). Des verres silicatés dopés avec 0.5 %pds de Fe2O3 ont été spécifiquement synthétisés dans des conditions d’oxydoréduction extrêmes, réductrices ou oxydantes. L’objectif étant d’isoler les signatures spectroscopiques respectives du Fe2+ et du Fe3+ qui sont habituellement mélangés en raison de la nature hétérovalente du fer. Cependant, les conditions de synthèse ne permettent pas d’isoler des environnements spécifiques du fer qui reste distribué dans les verres. Par conséquent, des minéraux contenant du Fe2+ et du Fe3+ dans des environnements bien définis ont été sélectionnés. La compréhension des relations structure-spectroscopie dans ces références cristallines est donc une étape préliminaire à l’interprétation des signatures spectroscopiques du fer dans les verres. Une étude multi-spectroscopique a été réalisée sur les verres et minéraux en combinant des méthodes expérimentales (absorption optique, absorption des rayons X au seuil K du fer, résonance paramagnétique électronique) et théoriques (calculs multiélectroniques dans l’approche en champ de ligand pour reproduire les spectres de pré-seuils K et de spectres optiques) pour mieux comprendre l’environnement local du fer dans les verres. L’étude des références cristallines a permis de montrer l’influence de la symétrie locale (Oh, Td, D4h/C4v et D3h/C3v) du site du fer et de l’hybridation p–d sur sa signature spectroscopique. Concernant l’étude des verres silicatés réduits, une majorité du Fe2+ est présent en coordinance 5, il est montré que cette dernière permet de reproduire et d’expliquer la signature du fer par une distribution des paramètres de champ cristallin rendant compte de la nature amorphe du verre. L’analyse des verres oxydés a permis d’appuyer l’existence de Fe3+ en coordinance 5 dans les verres sodo-calciques, dont la prise en compte est nécessaire pour l’interprétation des spectres optiques en complément des coordinances tétraédriques. Cette étude apporte un éclairage nouveau sur les variations structurales et spectroscopiques du fer en réaction à une substitution du calcium par du magnésium ou à l’absence d’alcalin dans la composition de la matrice verrière. / Among the infinite possibilities of glass compositions, soda-lime silicates represent of 90% of the glass production and are widely used for many industrial applications (e.g. automotive, solar panels, construction). Specific glasses silicate containing 0.5 wt% of Fe2O3 were synthesized with extreme redox. The use of reducing and oxidizing conditions allows to isolate the respective Fe2+ and Fe3+ spectroscopic signatures that are usually mixed due to the heterovalent nature of iron. However, synthesis conditions do not allow to isolate specific iron environment that remains distributed in glasses. Thus, Fe2+- and Fe3+-bearing minerals containing iron in well defined environments were selected. The comprehension of structure-spectroscopy relationships in these crystalline references is a preliminary step to the interpretation of iron spectroscopic signature in glasses. A multi-spectroscopic study was performed on glasses and minerals by combining experi- mental (optical absorption, X-ray absorption at the iron K edge, and electron paramagnetic resonance) and theoretical (multielectronic calculations using ligand field multiplet approach to reproduce the K pre-edge and optical absorption spectra) methods. The study of crystalline references evidenced the influence of local symmetry (Oh, Td, D4h/C4v et D3h/C3v) on iron sites and of p–d hybridization on iron spectroscopic signatures. Concerning the study of reduced silicate glasses, the majority of Fe2+ is present in 5- fold coordination, this speciation can reproduce and explain the iron signature by using a distribution of the crystal field parameters to take into account the amorphous nature of glass. The analysis of oxidized glasses supports the existence of Fe3+ in 5-fold coordination. These species need to be considered, in addition to tetrahedral geometries, for the interpretation of optical spectra. This study shed light on structural and spectroscopic variations of iron due to the substitution of calcium by magnesium or by the absence of alkali in the composition of the glass matrix.

Title: Magnetic excitations in cerium compounds Author: Kristina Vlášková Department: Department of Condensed Matter Physic Supervisor: doc. Mgr Pavel Javorský, Dr., Department of Condensed Matter Physics Abstract: Thanks to presence of only one electron in 4f shell of cerium ion, Ce based compounds reveal wide range of exceptional magnetic properties like valence fluctuations, magnetic ordering or spin glass behavior. In present thesis we mainly focus on tetragonal CeCuxAl4-x system and its magnetic behavior. Former results of inelastic neutron experiments opened discussion about energy levels of cerium ions in CeCuAl3. Except two crystal field excitations also one additional energy level was present, phenomenon of vibron quasi-bound state was proposed to explain observed energy structure. Measurement and analysis of specific heat and neutron inelastic scattering on compounds with various Cu content should help to understand phenomenon of vibron state and also its development with Cu/Al concentration. Another two types of cerium compounds will be investigated in means of specific heat and resistivity measurement, polycrystalline sample of CePt4Al and single crystal of Ce3Ru4Al12. Preparation, characterization and measurement of low temperature properties are subjects of this work. Results will be discussed...

In this work we report on physical properties of Nd2RhIn8 and Nd2IrIn8 rare-earth intermetallic compounds from the RmTnX{2n+3m} compound family. The compounds crystallize in the P4/mmm space group. Both compounds are anti-ferromagnetic with Néel temperatures TN of 10.8 K, 12.5 K respectively and their magnetization curves show characterisic step-like transitions at low temperatures. The measured M(T) and/or M(H) magnetization curves were used to determine magnetic phase diagrams for field along the c-axis. The specific heat of Nd2IrIn8 was analyzed at temperatures T<TN with respect to the dimensionality of the magnetic excitations. In the T>TN region the Schottky specific heat and susceptibility were compared to calculations based on the crystal field scheme in Nd2IrIn8 obtained by ab-initio methods. Powered by TCPDF (www.tcpdf.org)

<p> The magnetic susceptibility of samarium orthophosphate (SmPO4) has been measured over the temperature range from 0.4 to 270°K. A theoretical expression for the susceptibility has been developed assuming a crystal field of cubic symmetry and fitted to the experimental data. </p> / Thesis / Master of Science (MSc)