Yttrium garnet is an excellent laser host crystal due to its excellent optical, thermal, mechanical properties and high chemical stability. As a rare-earth ion with the simplest energy level construction, Yb3+ belongs to the 4f13 electronic configuration. It possess some important advantages such as long fluorescence lifetime, no excited-state absorption, low quantum defect and larger intrinsic laser slope efficiency. Besides, yttrium garnet crystal doping ytterbium ion can especially dope with high concentrations and lower heat generation. In this thesis, a series of different doping concentration of Yb¡GYAG by Czochralski pulling technique are grown and perform some accurate experiments including Raman, IR, XRD and EXAFS in order to make sure the impact effect of doping on the lattice structure. The study includes following ¡]1¡^the effect of different doping Yb3+ from pure YAG to YbAG¡]2¡^the distances of Yb-O, Yb-Al, Yb-Y, Yb-Yb when Yb3+ ions replace Y3+ ions¡]3¡^the changes of absorption energy level in YAG crystal due to different concentrations of Yb3+ ion.

Thèse de doctorat : Surfaces et interfaces : Paris 12 : 2003. / Version électronique uniquement consultable au sein de l'Université Paris 12 (Intranet). Titre provenant de l'écran-titre. Bibliogr. en fin de chapitres.

As computational methods for the prediction of metallic nanoparticle structure and reactivity continue to advance, a need has developed for simple experimental models that can mimic and confirm theoretical predictions. Dendrimer-encapsulated nanoparticles, or DENs, are ideal to fill this role. DENs are synthesized within poly(amido amine) dendrimer templates which allows for the controlled synthesis of monodisperse nanoparticles in the 50-250 atom (1-2 nm) size range. These are small enough to be accessible to high-level theoretical calculations while being large enough to study experimentally. The research reported here consists of several independent but closely related studies. First, the synthesis, structural, and electrochemical properties of Au@Pt (core@shell) DENs are described. These materials are prepared by underpotentially depositing Cu onto Au DENs followed by galvanic exchange of Cu for Pt. Second, Pb UPD onto Au DENs and a detailed experimental and theoretical study of the resulting core@shell particle structures and catalytic activity is discussed. It is found that no matter how much Pt is deposited onto the surface of Au₁₄₇ DENs, a surface reorganization occurs resulting in similar electrocatalytic activity for the oxygen reduction reaction. Third, an in-depth X-ray absorption spectroscopy study of the structural properties of thiol-capped Au₁₄₇ DENs is described. Here, the surfaces of uncapped Au₁₄₇ nanoparticles are titrated with strongly binding thiol ligands to tune the extent of surface disorder. The effect of the increased surface disorder on the standard EXAFS fitting results is discussed from experimental and theoretical perspectives. Lastly, an in-situ electrochemical study of Au₁₄₇ DENs structure is reported. The key result is that the Au lattice expands during electrochemical surface oxidation. This is an important result for understanding electrocatalytic processes on Au nanoparticle / text

Understanding the relationship between the structure of materials and their properties allows for the development of new applications and technologies. Here the relationship between local structure and optical properties in several binary oxide glass systems containing zinc oxide was examined, in particular the relationship between structure, applied stress and induced birefringence in a glass. The empirical model introduced by Zwanziger and co-workers posits a negative correlation between the ratio of the bond length to coordination number in a glass and the induced birefringence; zinc oxide in this model is predicted to be exactly at the threshold between positive and negative birefringence and is thus of particular interest to investigate. XAFS and Raman spectroscopy were used to determine local structure while the Sénarmont compensator method, Abbe refractometry and spectroscopic ellipsometry were used to measure optical properties.

Li-ion batteries have a huge potential for use in electrification of the transportation sector. The major challenge to be met is the limited energy storage capacity of the battery pack: both the amount of energy which can be stored within the space available in the vehicle (defining its range), and the aging of the individual battery cells (determining how long a whole pack can deliver sufficient energy and power to drive the vehicle). This thesis aims to increase our knowledge and understanding of structural changes induced by aging and usage of the Li-ion battery materials involved. Aging processes have been studied in commercial-size Li-ion cells with two different chemistries. LiFePO4/graphite cells were aged under different conditions, and thereafter examined at different points along the electrodes by post mortem characterisation using SEM, XPS, XRD and electrochemical characterization in half-cells. The results revealed large differences in degradation behaviour under different aging conditions and in different regions of the same cell. The aging of LiMn2O4-LiCoO2/Li4Ti5O12 cells was studied under two different aging conditions. Post mortem analysis revealed a high degree of Mn/Co mixing within individual particles of the LiMn2O4-LiCoO2 composite electrode. Structural changes induced by lithium insertion were studied in two negative electrode materials: in Li0.5Ni0.25TiOPO4 using in situ XRD, and in Ni0.5TiOPO4 using EXAFS, XANES and HAXPES. It was shown that Li0.5Ni0.25TiOPO4 lost most of its long-range-order during lithiation, and that both Ni and Ti were involved in the charge compensation mechanism during lithiation/delithiation of Ni0.5TiOPO4, with small clusters of metal-like Ni forming during lithiation. Finally, in situ XRD studies were also made of the reaction pathways to form LiFeSO4F from two sets of reactants: either FeSO4·H2O and LiF, or Li2SO4 and FeF2. During the heat treatment, Li2SO4 and FeF2 react to form FeSO4·H2O and LiF in a first step. In a second step LiFeSO4F is formed. This underlines the importance of the structural similarities between LiFeSO4F and FeSO4·H2O in the formation process of LiFeSO4F.

Stuttgart, Univ., Diss., 2001.

Stuttgart, Univ., Diplomarb., 2003.

Universiẗat, Diss., 1999--Duisburg.

O objetivo deste trabalho é estudar a dinâmica vibracional de dois materiais cristalinos: o germânio (Ge) e o arseneto de gálio (GaAs) por intermédio da técnica de XAFS. Inicialmente, uma breve revisão baseada nas teorias básicas de livro-texto de física é apresentada e seus resultados e simplificações discutidos com base no que se conhece atualmente sobre vibração dos átomos nos sólidos. Em seguida, a técnica de análise empregada neste trabalho, a estrutura fina estendida de absorção de raios X (EXAFS), é apresentada. É feita uma descrição pormenorizada dos parâmetros físicos que podem ser obtidos, bem como das diferenças que fazem do EXAFS uma técnica bastante sui generis. Após isso, dados de EXAFS da borda K do Ge, do gálio (Ga) e do arsênio (As), medidos como função da temperatura, no intervalo de 20-350 K, serão analisados. São obtidos, além de distâncias interatômicas e coeficientes de expansão térmica, fatores de Debye- Waller e os terceiros cumulantes das distribuições de distâncias para cada temperatura. Por meio destes resultados, puderam ser estimados coeficientes de expansão térmica linear e frequência de Einstein medidos por EXAFS. Os resultados obtidos neste trabalho são comparados, tanto qualitativamente quanto quantitativamente, com resultados já bem estabelecidos de outras medidas, por exemplo, difração de raios X (XRD), calorimetria e espalhamento inelástico de nêutrons. Uma maneira coerente de interpretar a frequência de Einstein é proposta com base em medidas de densidade de estados. Além disso, são expostas algumas limitações da técnica e do método de análise. / The main objective of this work is to study the vibrational dynamics of two crystalline materials: germanium (Ge) and gallium arsenide (GaAs) using EXAFS. Initially, a brief review of physic's textbook theories are presented and their results are discussed based on what is currently known about vibrations of crystals. Later, the theory of Extended X-ray Absorption Fine Structure (EXAFS) is presented. It is made a detailed description of the physical parameters that can be obtained as well as the di erences which make EXAFS a quite sui generis technique for solid state analysis. After that, EXAFS measurements of Ge, gallium (Ga) and arsenic (As) K edges carried out as function of temperature, in the range of 20-350 K, are analyzed. Interatomic distances, thermal expansions, Debye-Waller factors and the third cumulants of the distances distributions were obtained for each temperature. Through these results, linear thermal expansion coe cients and Einstein frequencies, as measured by EXAFS, could be estimated. The results obtained in this study were compared qualitatively and quantitatively with well-established results of measurements of other techniques, for example, X-ray di raction (XRD), calorimetry and inelastic neutron scattering. A consistent manner to interpret Einstein frequencies are proposed based on density of states measurements. Besides, some limitations, both of the technique and of the analysis method, are exposed.

O objetivo deste trabalho é estudar a dinâmica vibracional de dois materiais cristalinos: o germânio (Ge) e o arseneto de gálio (GaAs) por intermédio da técnica de XAFS. Inicialmente, uma breve revisão baseada nas teorias básicas de livro-texto de física é apresentada e seus resultados e simplificações discutidos com base no que se conhece atualmente sobre vibração dos átomos nos sólidos. Em seguida, a técnica de análise empregada neste trabalho, a estrutura fina estendida de absorção de raios X (EXAFS), é apresentada. É feita uma descrição pormenorizada dos parâmetros físicos que podem ser obtidos, bem como das diferenças que fazem do EXAFS uma técnica bastante sui generis. Após isso, dados de EXAFS da borda K do Ge, do gálio (Ga) e do arsênio (As), medidos como função da temperatura, no intervalo de 20-350 K, serão analisados. São obtidos, além de distâncias interatômicas e coeficientes de expansão térmica, fatores de Debye- Waller e os terceiros cumulantes das distribuições de distâncias para cada temperatura. Por meio destes resultados, puderam ser estimados coeficientes de expansão térmica linear e frequência de Einstein medidos por EXAFS. Os resultados obtidos neste trabalho são comparados, tanto qualitativamente quanto quantitativamente, com resultados já bem estabelecidos de outras medidas, por exemplo, difração de raios X (XRD), calorimetria e espalhamento inelástico de nêutrons. Uma maneira coerente de interpretar a frequência de Einstein é proposta com base em medidas de densidade de estados. Além disso, são expostas algumas limitações da técnica e do método de análise. / The main objective of this work is to study the vibrational dynamics of two crystalline materials: germanium (Ge) and gallium arsenide (GaAs) using EXAFS. Initially, a brief review of physic's textbook theories are presented and their results are discussed based on what is currently known about vibrations of crystals. Later, the theory of Extended X-ray Absorption Fine Structure (EXAFS) is presented. It is made a detailed description of the physical parameters that can be obtained as well as the di erences which make EXAFS a quite sui generis technique for solid state analysis. After that, EXAFS measurements of Ge, gallium (Ga) and arsenic (As) K edges carried out as function of temperature, in the range of 20-350 K, are analyzed. Interatomic distances, thermal expansions, Debye-Waller factors and the third cumulants of the distances distributions were obtained for each temperature. Through these results, linear thermal expansion coe cients and Einstein frequencies, as measured by EXAFS, could be estimated. The results obtained in this study were compared qualitatively and quantitatively with well-established results of measurements of other techniques, for example, X-ray di raction (XRD), calorimetry and inelastic neutron scattering. A consistent manner to interpret Einstein frequencies are proposed based on density of states measurements. Besides, some limitations, both of the technique and of the analysis method, are exposed.