Abstract
Local atomic and electronic structure investigations of uranium trioxide (UO3) crystalline phases performed by the U M4 edge HR-XANES technique is presented. The experimental U M4 edge HR-XANES spectra of α-UO3, β-UO3 and γ-UO3 polymorphic phases are compared with spectra of uranate (CaU2O7) and uranyl (UO3•1-2(H2O)) compounds. We describe a finger print approach valuable for characterization of variations of U-O axial bond lengths. Theoretical calculations of spectra using full-multiple-scattering theory (FEFF9.6 code) are performed. We have tested and selected input parameters, which provide best agreement between experimental and calculated spectra.
Highlights
Both structural and electronic properties of the uranium oxides are of fundamental and practical interest primarily due to the role of UO2 in the nuclear fuel cycle [1, 2]
The U M4 HR-XANES spectra of UO3 polymorphs are compared with the spectra of UO31-2(H2O) and CaU2O7
The A, B and C peaks have been assigned to electronic transitions of 3d3/2 electrons to 5f 5f (A), 5f (B) and 5f (C) unoccupied valence orbitals of U(VI) in the form of UO22+ [8]
Summary
Both structural and electronic properties of the uranium oxides are of fundamental and practical interest primarily due to the role of UO2 in the nuclear fuel cycle [1, 2]. The high energy resolution X-ray absorption near edge structure (HR-XANES) spectra at the U M4 edge (3d3/2 → 5f) have spectral features with reduced broadening compared to the conventionally measured XANES [7,8,9]. This advanced technique provides relative energy positions of valence unoccupied actinide (An) f orbitals not accessible by the conventional XANES method [7,8,9]. We present results of theoretical «ab-initio» full-multiplescattering (FMS) HR-XANES calculations for the α-UO3, β-UO3 and γ-UO3 phases by the FEFF 9.6 code [10]
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