Exploration Of Occupancy Of Light And Medium Atomic Lattices In UO₂and U₂N_3+xO_y

Thanks to its good mechanical performance and corrosion resistance,?-U₂N₃ can be a promising anti-oxidation layer for metallic uranium.However,oxygen in atmospheric environment is able to enter the ?-U₂N₃ lattice to form partly oxidized uranium oxy-nitride,U₂N_3+xO_y,and even oxidizes outmost few layers of ?-U₂N₃ to form UO2 film.But up to now,the underlying oxidation mechanism,light atoms' sites,level of oxidation,etc.in this phenomenon still lack sufficient attention.With respect to the very issue,this thesis interprets the near edge structure and the fine structure on the X-ray Absorption Spectra?XAS?and Electron Energy Loss Spectra?EELS?to gain the information of microstructure and electronic structure through various means of characterization,which provides a new approach of investigating oxidation of a-U₂N₃.To study U₂N_3+xO_y with more complex crystalline structure,and to verify that Extended Energy Loss Fine Structure?EXELFS?is applicable to uranium compounds,UO2 with relatively simple crystalline structure was selected for microstructural analysis by EXELFS.EXELFS was extracted from background-subtracted,deconvoluted oxygen-K edge by IFEFFIT package,and then was normalized into ??E?.After conversion of the energy E into wavevector k,??k?was obtained.Next,??k?was Fourier transformed into the r space to obtain the real-space experimental spectrum,i.e.radial distribution function?RDF?,which was then back-Fourier transformed into k space to get the reciprocal-space experimental spectrum,kn· ??k?.In the final step,these two experimental spectra were fitted by IFEFFIT package to obtain the information about local structure,e.g.distances between excited atom to its different coordination shells,coordination number and disorder of coordination atoms.To demonstrate the reliability of application of EXELFS to UO2,the lattice constant of UO2 determined by EXELFS analysis was compared with that obtained by X-Ray Diffraction?XRD?.The former was merely 1.4%lower than the latter.This confirms that EXELFS analysis is a feasible method of investigating UO2 microstructural information.Additionally,it was the first time to apply EXELFS analysis to uranium compound,laying the foundation for further investigation of other uranium compounds.For U₂N_3+xO_y,effort was made to utilize Extended X-ray Absorption Fine Structure?EXAFS?,very similar to EXELFS,to corroborate the nitrogen atom's sites in the classical model.Nonetheless,the lattice structure is more complex for U₂N_3+xO_y than that for Uo2,because in U₂N_3+xO_y lattice,there are two types of anions as well as two different U cations with different positions.Apart from EXAFS analysis,by analyzing the near edge structures,including Energy Loss Near Edge Structure?ELNES?and X-ray Absorption Near Edge Structure?XANES?collected by Transmission Electron Microscope?TEM?and synchrotron radiation respectively,for U₂N_3+xO_y and UO2,one can find similarities on their U-L3,N-K and O-K edges,which reflect the similarities in their lattice structures and site symmetry.Therefore,the most reasonable explanation is that rather than octahedral interstices,interstitial oxygen atoms are sited in tetrahedral interstices of ?-U₂N₃-interstitial oxygen atoms fill in the nitrogen vacancies in nonstoichiometric ?-U₂N₃.The U₂N_3+xO_y specimen was then exposed by electron beam for another 30 min.This electron irradiation brought difference to the near edge structure,which indicated the level of oxidation of U₂N_3+xO_y became deeper.It was also the first time to harness the near edge structure and the fine structure to identify the site of anions in U₂N_3+xO_y.