Apresentamos, pela primeira vez, estudos empregando Simulações Computacionais Estáticas e TSPC e TSDC de soluções sólidas de CaF2 com AIF (de 0,004 a 0,05 mol%), YbF3 e LaF? (- 0,l moi%). Os espectros de TSDC revelaram que: i) abaixo de 270K, as amostras só com Yb3+, La3+ e as multidopadas possuem a banda dos dipolos 1(0(0/11e dos defeitos 1j0/1/21, e as amostras pura ou só com A13+ não apresentam nenhuma banda; ii)acima de 270K, as amostras pura ou só com La3+ apresentam uma banda em 300K, atribuída a deslocações, e uma relaxação próxima a temperatura de polarização. As demais amostras apresentam uma banda predominante na faixa de 370 a 400K além desta última. TSPC combinado com TSDC mostrou que as bandas em 300K e entre 370 e 400K são devidas a fenômenos localizados de polarização e despolarização. Simulações computacionais mostraram que o A13+ não estabiliza dipolos isolados pois a energia de ligação dos agregados cubo-octaedrais é duas vezes menor do que a energia obtida para as terras raras. Nas soluções multidopadas, o AI3+ reduziu em mais de duas vezes a energia de ligação dos agregados cubo-octaedrais. As principais conclusões são: i)a banda entre 370 e 400K é devida ao agregado cubooctaedral; ii)o AI3+ gera preferencialmente agregados grandes do tipo cubo-octaedral; e iii)nas soluções multidopadas, o A13+ atua como \"indutor\" destes agregados. / In the present work, we report, for the first time, measurements of TSDC and TSPC combined with Static Computer Simulation in the CaF2 solid solutions with one or more of the fluorides of aluminium (concentration range between 0.004 and 0.05 mol%), yterbium and lanthanum (concentration of about 0.1 mol%). The following behavior was found in the TSDC spectra: i) below room temperature, the Yb3+ and La3+ and the multidoped samples revealed the 1101011 1 dipole band and the 110/1/21 band while the A13+ and the pure samples displayed no bands; ii) from room temperature, the La3+ and the pure samples showed a depolarization band at 300K due to dislocations, while all the other samples exhibited a band near 370-400K. Combining TSPC and TSDC it was possible to verify that both relaxation bands are due to some kind of localized polarization fenomena. The Computer Simulation results made clear that it is improbable to find isolated A13+ dipoles since the binding energy of the cubo-octahedral cluster is more than two times lower than the values obtained for the rare earth ions. Concerning to the multidoped systems, the binding energies of the mixed cubo-octaedral clusters with A13+ and one of the rare earth ions, were less than half of the values obtained for the same cluster with the rare earth ions. The main conclusions of this work can be sumarized as follows: i) the 370-400K band is due to the cubo-octaedral cluster; ii) the A13+ gives rise mainly to cubo-octahedral clusters; and iii) inside the multidoped samples, the A13+ promotes the formation of cubo-octaedral clusters with the rare earth ions.

Apresentamos, pela primeira vez, estudos empregando Simulações Computacionais Estáticas e TSPC e TSDC de soluções sólidas de CaF2 com AIF (de 0,004 a 0,05 mol%), YbF3 e LaF? (- 0,l moi%). Os espectros de TSDC revelaram que: i) abaixo de 270K, as amostras só com Yb3+, La3+ e as multidopadas possuem a banda dos dipolos 1(0(0/11e dos defeitos 1j0/1/21, e as amostras pura ou só com A13+ não apresentam nenhuma banda; ii)acima de 270K, as amostras pura ou só com La3+ apresentam uma banda em 300K, atribuída a deslocações, e uma relaxação próxima a temperatura de polarização. As demais amostras apresentam uma banda predominante na faixa de 370 a 400K além desta última. TSPC combinado com TSDC mostrou que as bandas em 300K e entre 370 e 400K são devidas a fenômenos localizados de polarização e despolarização. Simulações computacionais mostraram que o A13+ não estabiliza dipolos isolados pois a energia de ligação dos agregados cubo-octaedrais é duas vezes menor do que a energia obtida para as terras raras. Nas soluções multidopadas, o AI3+ reduziu em mais de duas vezes a energia de ligação dos agregados cubo-octaedrais. As principais conclusões são: i)a banda entre 370 e 400K é devida ao agregado cubooctaedral; ii)o AI3+ gera preferencialmente agregados grandes do tipo cubo-octaedral; e iii)nas soluções multidopadas, o A13+ atua como \"indutor\" destes agregados. / In the present work, we report, for the first time, measurements of TSDC and TSPC combined with Static Computer Simulation in the CaF2 solid solutions with one or more of the fluorides of aluminium (concentration range between 0.004 and 0.05 mol%), yterbium and lanthanum (concentration of about 0.1 mol%). The following behavior was found in the TSDC spectra: i) below room temperature, the Yb3+ and La3+ and the multidoped samples revealed the 1101011 1 dipole band and the 110/1/21 band while the A13+ and the pure samples displayed no bands; ii) from room temperature, the La3+ and the pure samples showed a depolarization band at 300K due to dislocations, while all the other samples exhibited a band near 370-400K. Combining TSPC and TSDC it was possible to verify that both relaxation bands are due to some kind of localized polarization fenomena. The Computer Simulation results made clear that it is improbable to find isolated A13+ dipoles since the binding energy of the cubo-octahedral cluster is more than two times lower than the values obtained for the rare earth ions. Concerning to the multidoped systems, the binding energies of the mixed cubo-octaedral clusters with A13+ and one of the rare earth ions, were less than half of the values obtained for the same cluster with the rare earth ions. The main conclusions of this work can be sumarized as follows: i) the 370-400K band is due to the cubo-octaedral cluster; ii) the A13+ gives rise mainly to cubo-octahedral clusters; and iii) inside the multidoped samples, the A13+ promotes the formation of cubo-octaedral clusters with the rare earth ions.

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<p> The melt equilibria of the system CaF2-CaMgSi2O6 has been studied at atmospheric pressure by using a modified quenching method. This system is characterized by a simple binary eutectic at CaF2 43.4, CaMgSi2O6 56.5 weight percent at 1082t 2°C; neither solid solution nor intermediate compound was found. Some special features have been discussed in detail. The results of the study of the system CaF2-CaMgSi2O6, together with suppositions regarding the system CaF2-CaMgSi2O6-CaCO3, have tentatively been applied to a hypothesis regarding the origin of certain skarns which are considered to be formed by differential melting of impure limestone in regional metamorphic terrains. The applications of this binary system to the theoretical chemistry of Portland cement burning is also incidentally considered. </p> / Thesis / Master of Science (MSc)

Ce travail de thèse concerne le développement de sources femtoseconde de forte puissance moyenne ou de forte énergie avec des matériaux pompés par diodes laser, dopés à l’ytterbium. Plus particulièrement au cours de cette thèse deux types de matrices ont été utilisés, le CALGO (CaGdAlO4) et les fluorures, possédant le potentiel de générer des impulsions courtes (100aine de femtoseconde). Les caractéristiques spectroscopiques et thermiques du CALGO dopés à l’ytterbium permettent d’envisager le développement d’oscillateur femtoseconde court de forte puissance moyenne. Dans ce contexte, la technologie des disques minces permet d’obtenir avec d’autres matrices, des résultats très intéressants. C’est pourquoi durant cette thèse le choix de maitriser cette nouvelle technologie, avec l’utilisation de ce cristal, a été fait. Dans ce cadre, des résultats très prometteurs ont été obtenus. L’oscillateur femtoseconde Yb :CALGO de plus forte puissance moyenne a en effet été développé (28 W), pour une énergie non négligeable, supérieure au µJ et une durée d’impulsions de 300 fs. Des améliorations sont à prévoir avec l’utilisation de nouveaux cristaux plus dopés et plus fins, mais d’hors et déjà les résultats obtenus sont au niveau de l’état de l’art des oscillateurs femtoseconde de forte puissance moyenne.Le cristal de CaF2 quant à lui, possède un grand intérêt pour le développement d’amplificateurs énergétiques courts, puisqu’il a la capacité de stocker beaucoup d’énergie. Deux types d’amplificateurs ont alors été développés, avec des objectifs bien différents. Le premier permet d’obtenir un fort gain (~106), avec une énergie extraite proche du mJ (amplificateur régénératif), alors que le second a pour but d’extraire le maximum d’énergie (amplificateur multipassage), dans notre cas jusqu’à 160 mJ, avec un gain plus faible (~10).Le potentiel de ces matériaux pour la génération d’impulsions courtes et/ou de forte puissance moyenne a alors été démontré. / This work concerns the development of high average power or high energy laser with diode-pumped ytterbium-doped materials. Two host matrices were studied, CALGO (CaGdAlO4) and fluoride, which permit the generation of ultra-short pulses. Spectroscopic and thermal properties of ytterbium doped CALGO crystals are adapted for the development of high average power oscillator. In this area, thin disk technology seems to be particularly interesting for the development of such oscillator. That’s why we choose to master this technology with ytterbium-doped CALGO crystals. Thus, Yb:CALGO oscillator with the highest average power was developed (28 W), with more than µJ energy and pulse duration of 300 fs. Using more doped and thinner crystals should permit to improve our performances, however they are already at the state of the art of high average power oscillator.Ytterbium doped CaF2 has a great interest for short-pulse high energy amplifier, thanks to its capacity to store energy. Two types of amplifier were developed. A regenerative amplifier with high gain (~106), mJ energy level, and a multipass amplifier with lower gain (~10) but permitting to extract really high energy (up to 160 mJ).Potential of these materials for the development of short pulse high average power and/or high energy system was demonstrated.

Ce travail de thèse a pour objectif l'étude du frittage ainsi que l'étude des propriétés spectroscopiques et lasers de céramiques transparentes de fluorure de calcium dopé par des ions ytterbium pour une application comme matériau amplificateur pour les lasers de puissance. Dans le premier chapitre, nous présentons les avantages des céramiques transparentes sur les verres et les monocristaux tant du point de vue des propriétés thermiques et mécaniques que du point de vue du procédé d'élaboration. Dans une deuxième partie, nous décrivons le protocole expérimental d'élaboration de céramiques transparentes de fluorure de calcium dopé par les ions ytterbium ainsi que des améliorations possibles afin de diminuer les pertes optiques. L'étude de l'origine des pertes optiques par microscopie électronique a mis en évidence la présence de défauts au niveau des joints de grains. Dans le troisième chapitre, nous étudions les propriétés spectroscopiques à 20 K des ions ytterbium dans nos céramiques et analysons des images issues de la microscopie électronique pour mettre en évidence la présence d'ions ytterbium sous forme de clusters, espèces responsables de la fluorescence. Dans le dernier chapitre, nous nous sommes intéressés aux propriétés spectroscopiques des céramiques transparentes à 77 K, température d'utilisation envisagée, et à la démonstration de l'effet laser dans nos céramiques.

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.

Impurity trapped excitons (ITEs) occurring in divalent ytterbium doped calcium and strontium fluoride crystals have been investigated by exploiting the radically different radiative decay rates of the lowest exciton state and higher excited states, utilizing a novel two-colour transient photoluminescence enhancement experiment. The ITE energy levels have been directly measured with the observation of sharp transitions occurring from the changes of states of the localized hole and broad bands associated with changes of state of the delocalized electrons. The dynamic behaviour under excitation by time delayed ultra-violet (UV) and infrared (IR) pulses has been observed allowing for the identification of excitation and decay pathways between the ITE states. The position and transition intensities of the sharp lines within the IR excitation spectrum have been successfully matched using a semi-empirical effective Hamiltonian crystal field model. In CaCaF₂:Yb²⁺ the lines occurring at 249 and 1145 cm⁻¹ were matched with the crystal field parameter B4 = 800 cm⁻¹ and the exchange parameter G3(fs) = 5900 cm⁻¹. In SrCaF₂:Yb²⁺ these lines were observed at 178 and 1284 cm⁻¹ and matched with B4 = 600 cm⁻¹ and G3(fs) = 7278 cm⁻¹. Local heating and direct absorption by intra-excitonic transitions are found to be the causes of the broad band observed in the spectrum and have been deconvolved by studying the dynamic behaviour of the monitored emission at different IR excitation frequencies. Through this modeling, higher lying ITE states have been identified occurring at 785 cm⁻¹ in SrCaF₂:Yb²⁺ and in between 740 - 820 cm⁻¹ in CaCaF₂:Yb²⁺. The dynamic model developed successfully simulates the temporal behaviour of the emission under IR excitation under a variety of parameters including IR fluence, excitation frequency, sample temperature and UV - IR pulse delay. Examination of the SrCaF₂:Yb²⁺ dynamic behaviour over a time scale of 100 ms shows UV driven trap population at a rate of approximately 3% per pulse, which are liberated and recycled to the Yb²⁺ ground state by the IR pulse. The two-colour technique is applied to MgCaF₂:Yb²⁺, a candidate for possible ITE emission. Temperature dependent emission spectra obtained under UV excitation indicates the possibility of an ITE state, independently populated from the 5d level of the Yb²⁺. Typical 5d emission is also observed from this system. Under IR excitation, liberation of shallow traps and possible local heating is observed. No ITE emission is conclusively found with IR probing.

La possibilité d’obtenir une émission laser efficace sur une large bande spectrale autour de 1.05 µm a été démontrée récemment en utilisant des monocristaux CaF2 and SrF2 dopés Nd3+. Cette émission, éteinte du fait de la relaxation croisée entre ions dans les matériaux de type fluorite, peut être exaltée avec l’insertion d’ions “tampons” comme Y3+ ou Lu3+. Une émission laser accordable sur une large bande spectrale et la génération d’impulsions ultracourtes sont donc possibles. De plus, ce type de matériau est particulièrement intéressant pour des amplificateurs de forte puissance pompés par diodes. Une étude complète des propriétés spectrales et thermomécaniques de cristaux de CaF2:Nd3+ co-dopés avec des concentrations variables de Lu3+ a été menée en collaboration avec le CEA CESTA au sein du projet LASCAN pour de futurs développements au sein du Laser MégaJoule (LMJ) avec pour but l’amélioration des performances des amplificateurs pilotes. L’effet d’élimination des clusters de Nd3+ par le co-dopage avec le Lu3+ et l’apparition de deux sites actifs différents a été mise en évidence et caractérisée en détail. Ce travail présente ainsi une spectroscopie complète des centres actifs dans CaF2:Nd3+,Lu3+, une étude des propriétés thermomécaniques, ainsi que des propriétés d’amplification optique des matériaux étudiés. / It was recently demonstrated that efficient and broadband laser emission was possible with Nd-doped CaF2 and SrF2 single crystals around 1.05 µm. Such laser emission, known as completely quenched because of cross-relaxation in the singly doped fluorites, increases spectacularly by co-doping with non-optically active “buffer” ions like Y3+ or Lu3+. Broadband laser emission and ultra-short laser operation are therefore possible. The material is particularly appealing for large scale high peak power diode-pumped amplifiers. A deep investigation of the spectral properties of CaF2:Nd3+ crystals co-doped with various amounts of Lu3+ was performed. The cluster breaking effect of Lu3+ codoping has as a side-effect the appearance of two different contributions, identified as two different active sites, which is in agreement with recent observations. In this work, we investigate in depth the different centres’ spectroscopic properties and operational parameters for laser applications,their evolution as the lutetium concentration increases, and the thermomechanical properties of said samples. A preliminary investigation on the amplification properties and laser operation of said materials is performed as well.

It is accepted worldwide that the best final solution for spent nuclear fuel is to bury it in deep geological repositories. Despite the physical and chemical barriers that are supposed to isolate the nuclear waste for at least 100.000 years, some uncertainty factors may cause underground water to get in contact with the nuclear waste. Due to radioactivity and oxidation under air, dissolution experiments using UO2 pellets are difficult and frequently lead to incoherent results. Therefore, to enable a detailed study of the influence of microstructure and surface properties on the stability of spent nuclear fuel over time, it is necessary to produce analogues that closely resemble nuclear fuel in terms of crystallography and microstructure. At the same time, in-depth understanding of dissolution phenomena is crucial to geological processes such as dissolution precipitation creep and solvent mediated phase transformations. My thesis is based in two manuscripts. Paper I reports the microstructures obtained after sintering CaF2 powders at temperatures up to 1240°C. Pellets with microstructure, density and pore structure similar to that of UO2 spent nuclear fuel pellets were obtained in the temperature range between 900°C and 1000°C. Paper II reports how differences of surface chemistry and crystal symmetry, characteristics of each surface orientation, affect the topography of CaF2 pellets described in paper I during dissolution. I propose that every orientation of the fluorite structure can be decomposed in the three reference surfaces {100}, {110} and {111}. The {111} is the most stable surface with a dissolution rate of the top surface of 1,13x10-9 mol.m-2.s-1, and {112} the less stable surface with a dissolution rate 34 times faster that {111}. Surfaces that expose both Ca and F atoms in the same plan dissolve faster, possibly because the calcium is more susceptible to be solvated. The faster dissolving surfaces are replaced by the more stable {111} and {100} surfaces which causes the development of roughness on the top surface and stabilizes the surface on high energy sites; i.e. pores or grain boundaries. The main consequences of these observations are i) the increase of the total surface area; ii) the decrease of the overall surface energy. I present a dissolution model for surfaces of crystal with different surface energies. The main conclusions are: a) dissolution rates calculated from surface area are over estimated to the real dissolution rate; b) dissolution rates are faster at the beginning of dissolution and tend to diminish with time until a minimum value is reached.