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High energy resolution X-ray spectroscopy of f-electron systems

Abstract : X-rays emitted by synchrotrons are great for studying the atomic structure of materials due to their penetrative nature, as well as their sensitivity to the local and electronic structures of the selected element. It is significantly important for f-elements, precisely actinides since synchrotron-based methods are non-distractive. Actinides such as uranium and plutonium are essential due to their use as a nuclear fuel in commercial nuclear reactors worldwide. Therefore the release of uranium, plutonium and the products of their fission in the environment is of significant concern. The chemical complexity of these elements as well as low concentrations in the samples and/or high radioactivity demand advanced physical methods to reveal fundamental and complicated properties of f-elements, which are needed to predict long-term release from underground repositories of nuclear waste and contaminated sites. In this PhD thesis, the application of various synchrotron-based methods, especially high energy resolution X-ray spectroscopy, on systems containing f-elements is shown.The first part is devoted to synchrotron physics. Beamline setup is discussed in detail using the example of the Rossendorf Beamline II (ROBL-II) at ESRF. General information about the beamline as well as peculiarities associated with the study of radioactive materials are explained in details. Hutch arrangement and full descriptions of experimental stations including experimental equipment are covered.The second part is dedicated to a detailed description of various synchrotron methods such as X-ray diffraction, X-ray absorption near edge structure (XANES) spectroscopy, high energy resolution fluorescence detection (HERFD) XANES, extended X-ray absorption fine structure (EXAFS) spectroscopy and high-energy X-ray scattering (HEXS). The theoretical basis for these methods and their underlying fundamental physics principles are considered. Special attention is given to the HERFD method for this study. The experimental setup used in each method and the most important characteristics are described. All methods will be applied to actinide systems, and a real application of each method will be presented.In particular, the formation processes of MeO2 (Me = Ce, Th, U, Pu) nanoparticles (NPs) and their characterization are discussed. The following parameters of the synthesis are varied: initial oxidation state of the element in which it is taken for the experiment, its concentration, pH, temperature. The structural stability of NPs in terms of time, as well as redox transformation, and thermal sensitivity are also investigated. Pu containing NPs obtained from plutonium solutions of different oxidation states and at various pH are probed for their structural and electronic properties. The presence of oxidation state impurities is investigated. The intermediate phase of Pu(V) in the course of the PuO2 NPs synthesis is described, local structure and the oxidation state of this compound are examined. Besides PuO2 NPs, their analogs – UO2, CeO2 and ThO2 – have been investigated, the results are compared to PuO2. The most important peculiarities for each system have been highlighted. The formation of UO2 NPs under inert conditions is observed, and their oxidation after time under an X-ray beam is remarked. It is found that the properties of CeO2 NPs are affected by their size and their surface is modified by drying at different temperatures. The formation of ThO2 NPs is studied, nanoparticle size was extracted from the HEXS data. HEXS data are fitted by semi-empirical methods and fits based on real NP structures and reveal the presence of small and medium NPs depending on the synthesis conditions. The size effect of ThO2 NPs is also determined.
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Submitted on : Friday, December 3, 2021 - 2:43:08 PM
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Evgeny Gerber. High energy resolution X-ray spectroscopy of f-electron systems. Materials Science [cond-mat.mtrl-sci]. Université Grenoble Alpes [2020-..], 2021. English. ⟨NNT : 2021GRALY023⟩. ⟨tel-03464806⟩

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