Les propriétés électroniques de (Ga,Mn)N ont été étudiées par spectroscopie d'absorption des rayons X au seuil K du Mn. Des calculs ab-initio ont été utilisés pour interpréter les spectres d'absorption de (Ga,Mn)N. Deux pré-pics sont présents dans le seuil du Mn: le premier pré-pic est attribué aux transitions électronique vers les états 3d du Mn de spin up, tandis que le second pré-pic correspond aux transitions vers les états 3d du Mn de spin down. Cette interprétation nous permet de déterminer que l'état électronique du Mn dans (Ga,Mn)N est Mn³⁺: deux pré-pics sont présents dans les spectres d'absorption du Mn³⁺ et un seul pré-pic reste dans les spectres du Mn²⁺. Ce changement des spectres a été vérifié expérimentalement sur des échantillons de (Zn,Mn²⁺)Te et (Ga,Mn²⁺)As. De plus, cette interprétation permet d'étudier la distribution du Mn dans (Ga,Mn)N: la forme des spectres d'absorption suggère que la distribution du Mn est homogène dans nos échantillons de (Ga,Mn)N.

Pyrochlore-type oxides (A2B2O7) and glass-ceramic composites have been investigated for nuclear waste sequestration applications due to the remarkable compositional diversity and structural flexibility of these materials. These properties can enhance the incorporation of radioactive waste elements and resistance to radiation induced structural damage. Radiation induced structural damage can be simulated by bombarding materials using high-energy heavy ions. The study has shown how the metal-oxygen bond covalency, cationic radii ratio (rA/rB), and oxygen vacancies of pyrochlore type oxides affect the resistance of these materials to radiation induced damage. RE2Ti2O7 (RE=La–Lu), Yb1.85Ca0.15Ti2O7-δ, Yb2Ti1.85Fe0.15O7-δ, and Gd2Ti2-xSnxO7 were synthesized by the ceramic method and investigated by X-ray absorption near edge spectroscopy (XANES), which allows for the study of the effect of elemental substitution on the electronic structure of materials. Surface sensitive glancing angle and total electron yield XANES (GA/TEY XANES) spectra have been used to study the damaged surface of the materials, as the high energy ions can only implant in the near-surface region (~ 450 nm) of the pellets. These measurements have allowed for an investigation of how the local structure of the materials changed after ion implantation and discussed in terms of coordination number and bonding environment. After investigating the ceramic materials, the glass-ceramic composite materials containing Gd2Ti2O7 pyrochlore type crystallites in a (borosilicate- and Fe-Al-borosilicate) glass were investigated. These glass-ceramic materials were synthesized and analyzed by backscattered electron (BSE) images and XANES spectra. The study has shown how the Gd2Ti2O7 crystallites interact within a glass matrix depending on glass composition, pyrochlore loading, and annealing temperature. Further, the GA-XANES spectra from these materials have shown that the glass ceramic composite materials show a similar response to ion implantation as pure ceramics (i.e., Gd2Ti2O7). All of these studies and techniques could provide a better understanding of how to develop and design materials for nuclear waste sequestration applications.

No description available.

Although fluoride is used at low concentrations in drinking water as a means of promoting dental health, it poses a danger at high exposure levels where it can lead to skeletal fluorosis or other adverse effects. Cyanide is notoriously toxic, and its large scale use in industrial processes warrants the need for close monitoring to remain aware of potential contamination of water sources and other environmental resources. Based on these considerations, it is critical to continue to develop improved methods of monitoring fluoride and cyanide concentrations in water. However, molecular recognition of these anions in water poses considerable challenges. For fluoride, this is due largely to its high hydration enthalpy (Ho = -504 kJ mol-1), which drastically reduces its reactivity in water. Additionally, the strong basicity of cyanide (pKa of (HCN) = 9.3) may obscure its detection in neutral water due to protonation. In addition to achieving detection of these anions in water, it is most desirable to have information of the detection event relayed in the form of a positive, rather than negative, response (i.e., turn-on vs turn-off). The general strategy of appending cationic groups to triarylboranes imparts beneficial Coulombic, inductive, and sometimes chelate effects that have allowed a number of these Lewis acidic receptors to sense fluoride and cyanide in aqueous environments. With the goal of developing new triarylborane-based receptors that show enhanced affinities for these anions, as well as turn-on responses to detection, a series of pyridinium boranes were synthesized and studied. Having recognized that the inherent Lewis acidity of antimony(V) species might be exploited for anion sensing, we also describe initial studies on the ability of tetraorganostibonium ions (R4Sb+) and cationic transition metal-triarylstibine complexes (R3SbM+) to complex fluoride. Finally, the electropositivity of antimony and its ability to form stable compounds in both the +3 and +5 oxidation states have led us to begin investigations into the bonding and redox reactivity of novel metal stibine/stiborane complexes.

Despite the importance of soil organic matter (SOM) in C storage and provision of ecosystem services, the magnitude and direction of the response of SOM to climate change remains debated. Particularly contested is the role of biochemical recalcitrance in determining the biological stability of SOM, which in turn, may also vary with climate. Employing a climosequence study design controlling for confounding pedogenic factors, the research described in this thesis aimed to uncover the response of both SOM chemistry and SOM biological stability to changes in climate and associated land use shifts at the grassland-forest ecotone in west-central Saskatchewan. Characterization of SOM chemistry was achieved using two advanced analytical techniques: X-ray absorption near edge structure (XANES) spectroscopy and pyrolysis-field ionization mass spectrometry (Py-FIMS). Agreements between XANES and Py-FIMS revealed only minor differences in SOM chemistry resulting from a 0.7 °C mean annual temperature (MAT) gradient and associated broad differences in land use, but revealed a clear influence of depth within soil profiles. In contrast, long-term aerobic incubations revealed that biological stability of SOM varied with both climate and climate-induced differences in land use, but was not largely influenced by depth. Together, these findings suggest a decoupling of SOM chemistry and its biological stability, indicating that factors other than biochemical recalcitrance are the primary drivers of SOM persistence in these soils.

Doctor of Philosophy(PhD) / This project focuses on the examination of the structures of lanthanide containing double perovskites of the type Ba2LnB'O6-d (Ln = lanthanide or Y3+ and B' = Nb5+, Ta5+, Sb5+ and/or Sn4+) using synchrotron X-ray and neutron powder diffraction. The first part of this project examined the relative stability of R3 rhombohedral and I4/m tetragonal structures as the intermediate phase adopted by the series Ba2LnB'O6 (Ln = lanthanide (III) or Y3+ and B' = Nb5+, Ta5+ or Sb5+). It was found that I4/m tetragonal symmetry was favoured when B' was a transition metal with a small number of d electrons, such as Nb5+ or Ta5+. This is due to the presence of p-bonding in these compounds. In the Ba2LnNbO6 and Ba2LnTaO6 series R3 rhombohedral symmetry was, however, favoured over I4/m tetragonal symmetry when Ln = La3+ or Pr3+ due to the larger ionic radius of these cations. The incompatibility of the d0 and d10 B'-site cations in this family of compounds was indicated by significant regions of phase segregation in the two series Ba2Eu1-xPrxNb1-xSbxO6 and Ba2NdNb1-xSbxO6. In the second part of this project the compounds in the series Ba2LnSnxB'1-xO6-d (Ln = Pr, Nd or Tb and B' = Nb5+ or Sb5+) were examined to understand the relative stability of oxygen vacancies in these materials compared to the oxidation of the lanthanide cations and to determine if any oxygen vacancy ordering occurred. It was found, using a combination of structural characterisation, X ray Absorption Near Edge Structure and Ultra-Violet, Visible and Near Infrared spectroscopies, that with Ln = Pr or Tb increased Sn4+ doping results in a change in the oxidation state of the Ln3+ cations to Ln4+. This leads to those series containing little or no oxygen vacancies. A loss of B site cation ordering was found to accompany this oxidation state change and phase segregation was found to occur in the Ba2PrSnxSb1-xO6-d series most likely due to the Pr3+ and Pr4+ cations segregating into different phases. The Nd3+ cations in the series Ba2NdSnxSb1-xO6-d, however, can not oxidise to the tetravalent state so the number of oxygen vacancies rises with increasing x. It was found that oxygen vacancies concentrate onto the axial site of the compounds with x = 0.6 and 0.8 at ambient temperature. In Ba2Sn0.6Sb0.4O5.7 the oxygen vacancies were found to change to concentrating on the equatorial site at higher temperatures and it is suggested that this oxygen vacancy ordering plays a role in the adoption of I2/m monoclinic symmetry.

Techn. Universiẗat, Diss., 2001--Darmstadt.

Doctor of Philosophy / Department of Agronomy / Ganga M. Hettiarachchi / Constructed wetland treatment systems (CWTSs) are an alternative adaptation for flue-gas desulfurization (FGD) wastewater purification. A series of laboratory-based soil column studies mimicking a pilot-scale CWTS was carried out to evaluate the performance of the treatment system in detail. The main objectives of studies were to (1) understand the transport characteristics, retention capacity and transformation of selenium and other FGD constituents in the CWTS, (2) evaluate the effectiveness of soil treatments and influent flow rate on the performance of the CWTS, and (3) develop a mechanistic understanding of the CWTS performance through monitoring interrelationships of selenium (Se), arsenic (As), iron (Fe), and sulfur (S). Ferrihydrite (1% w/w), and labile organic carbon (OC) were used as soil treatments. Different influent flow rates, X (1.42 mL/hour), 2X, or 1/2X were used depending on the objectives of each study. Deoxygenated 1:1 mixture of FGD: raw water was the influent. It was delivered to the saturated columns with an upward flow. Effluent samples were collected continuously, and analyzed for constituents of concern. End of these experiments, soil from sectioned columns were used for total elemental analysis, sequential extraction procedure (SEP) for Se, and synchrotron-based X-ray spectroscopy analyses. Results indicated a complete Se retention by the columns. Boron, and fluorine partially retained whereas sodium, sulfur, and chlorine retention was weak, agreeing with field observations. Some of the initially-retained Se (~ 4 to 5%) was mobilized by changing redox conditions in the soil. Selenium fed with the wastewater accumulated in the bottom 1/3 (inlet) of the soil columns and was mainly sequestrated as stable forms revealed by SEP. Bulk-, and micro-XANES analyses suggested the retention mechanism of Se from the FGD wastewater was via the transformation of Se into reduced/stable forms [Se(IV), organic Se, and Se(0)]. Under wetland conditions, native soil As was mobilized by reductive dissolution of As associated minerals. However, the ferrihydrite amendment suppressed the native soil As mobility. Micro-XRF mapping integrated with As, and Fe-XANES suggested that the mechanism of native soil As retention was the sequestration of released As with newly precipitated secondary Fe minerals. A long-term study carried out with X, 1/2X flow rates, and OC source indicated enhanced S retention by the slow flow rate (1/2X), most likely due to the time dependency of biogenic S reduction. Further, bulk S-, As-, and Fe-XANES revealed that long submergence period and the slow flow rate increased the formation of reduced and/or biogenic S, realgar-like, and greigite-like species. These observations indicated that modified flow rates could have a significant impact on the long-term trace element (such as As) sequestration in the CWTS. Our studies provide useful information to improve the performance, and longevity of a full-scale CWTS for FGD wastewaters.

Argon oxygen decarburization slag (AOD) was tested in batch and column experiments to investigate its phosphorus (P) removal performance. The effects of factors such as AOD dose, initial P concentration of the feeding solution, and aging on the P removal ability of the slags were analyzed. In a column experiment, electric arc furnace slag (EAF), blast furnace slag (BFS) and AOD were combined in five different ways to determine optimal conditions for P removal. In another column experiment, the three types of slag were modified with polyethylene glycol (PEG) and NaOH to adjust their dissolution properties and the effect on P removal performance was examined. In the batch experiments, AOD exhibited very promising P removal ability. It removed 94.8% of P from 6.5 mg P L-1 synthetic solution in 4 hours with a dose of 5 g L-1. Maximum P removal capacity of 27.5 mg P g-1 was achieved. In the dual-filter column experiment, the column packed with only EAF had the best P removal performance (consistently above 93%). Amorphous calcium phosphate (ACP) was identified as the main P species in the five slag samples collected from the outlet chambers. The contributions from crystalline calcium phosphate (Ca-P) and P adsorbed on iron/aluminum (hydr)oxides were greater in samples from the inlet chambers. The P speciation results revealed that P was predominantly removed by the slags through formation of ACP. The second column experiment showed that modification with PEG and NaOH solution only enhanced short-term P removal by the slags. However, exhaustion of the modified slags occurred much earlier, indicating that the modification process had shortened the lifespan of the slags. Untreated AOD showed better P removal than untreated EAF until pore volume 244, probably due to faster dissolution rate of gamma dicalcium silicate (dominating in AOD according to the XRD results) than of beta dicalcium silicate (dominating in EAF). / <p>QC 20170830</p>

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.