Electronic structure of americium sesquioxide probed by resonant inelastic x-ray scattering

The Am $5d$-$5f$ resonant inelastic x-ray scattering (RIXS) data of americium sesquioxide were measured at incident photon energies throughout the Am $O_{4,5}$ edges. The experiment was supported by calculations using several model approaches. While the experimental Am $O_{4,5}$ x-ray absorption spectrum of Am$_2$O$_3$ is compared with the spectra calculated in the framework of atomic multiplet and crystal-field multiplet theories and Anderson impurity model (AIM) for the Am(III) system, the recorded Am $5d$-$5f$ RIXS data are essentially reproduced by the crystal-field multiplet calculations. A combination of the experimental scattering geometry and theoretical analysis of the character of the electronic states probed during the RIXS process confirms that the ground state of Am$_2$O$_3$ is singlet $\Gamma_1$. An appearance of the low-intense charge-transfer satellite in the Am $5d$-$5f$ RIXS spectra at an energy loss of $\sim$5.5 eV, suggests weak Am $5f$-O $2p$ hybridization which is in agreement with AIM estimations of the $5f$ occupancy from spectroscopic data in Am$_2$O$_3$ as being 6.05 electrons

X-ray spectroscopic study of chemical state in uranium carbides

UC and UMeC₂ (Me = Fe, Zr, Mo) carbides were studied by the high-energy-resolution fluorescence-detected X-ray absorption (HERFD-XAS) technique at the U M₄ and L₃ edges. Both U M₄ and L₃ HERFD-XAS reveal some differences between UMeC₂ and UC; there are differences also between the M₄ and L₃ edge results for both types of carbide in terms of the spectral width and energy position. The observed differences are attributed to the consequences of the U 5f, 6d–4d(3d) hybridization in UMeC₂. Calculations of the U M₄ HERFD-XAS spectra were also performed using the Anderson impurity model (AIM). Based on the analysis of the data, the 5f occupancy in the ground state of UC was estimated to be 3.05 electrons. This finding is also supported by the analysis of U N₄,₅ XAS of UC and by the results of the AIM calculations of the U 4f X-ray photoelectron spectrum of UC

Effect of carbon content on electronic structure of uranium carbides

The electronic structure of UC$_x$ (x = 0.9, 1.0, 1.1, 2.0) was studied by means of x-ray absorption spectroscopy (XAS) at the C K edge and measurements in the high energy resolution fluorescence detection (HERFD) mode at the U M$_4$ and L$_3$ edges. The full-relativistic density functional theory calculations taking into account the Coulomb interaction U and spin-orbit coupling (DFT+U+SOC) were also performed for UC and UC$_2$. While the U M$_4$HERFD-XAS spectra of the studied samples reveal little difference, the U HERFD-XAS spectra show certain sensitivity to the varying carbon content in uranium carbides. The observed gradual changes in the U M$_4$ HERFD spectra suggest an increase in the C 2p-U 5f charge transfer, which is supported by the orbital population analysis in the DFT+U+SOC calculations, indicating an increase in the U 5f occupancy in UC as compared to that in UC. On the other hand, the density of states at the Fermi level were found to be significantly lower in UC$_2$, thus affecting the thermodynamic properties. Both the x-ray spectroscopic data (in particular, the C K XAS measurements) and results of the DFT+U+SOC calculations indicate the importance of taking into account U and SOC for the description of the electronic structure of actinide carbides

Insight into the structure-property relationship of UO2_{2} nanoparticles

Highly crystalline UO$_{2}$ nanoparticles (NPs) with sizes of 2–3 nm were produced by fast chemical deposition of uranium(IV) under reducing conditions at pH 8–11. The particles were then characterized by microscopy and spectroscopy techniques including high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), high-energy resolution fluorescence detection (HERFD) X-ray absorption spectroscopy at the U M$_{4}$ edge and extended X-ray absorption fine structure (EXAFS) spectroscopy at the U L$_{3}$ edge. The results of this investigation show that despite U(IV) being the dominant oxidation state of the freshly prepared UO$_{2}$ NPs, they oxidize to U$_{4}$O$_{9}$ with time and under the X-ray beam, indicating the high reactivity of U(IV) under these conditions. Moreover, it was found that the oxidation process of NPs is accompanied by their growth in size to 6 nm. We highlight here the major differences and similarities of the UO$_{2}$ NP properties to PuO$_{2}$, ThO$_{2}$ and CeO$_{2}$ NPs

Signatures of technetium oxidation states: a new approach

A general strategy for the determination of Tc oxidation state by new approach involving X-ray absorption near edge spectroscopy (XANES) at the Tc L-3 edge is shown. A comprehensive series of Tc-99 compounds, ranging from oxidation states I to VII, was measured and subsequently simulated within the framework of crystal-field multiplet theory. The observable trends in the absorption edge energy shift in combination with the spectral shape allow for a deeper understanding of complicated Tc coordination chemistry. This approach can be extended to numerous studies of Tc systems as this method is one of the most sensitive methods for accurate Tc oxidation state and ligand characterization

Intrinsic deviations in fluorescence yield detected x-ray absorption spectroscopy : the case of the transition metal L-2,L-3 edges

Fluorescence yield (FY) detected x-ray absorption spectra (XAS) of 3d transition metal ions are calculated from the integrated 2p3d resonant x-ray emission spectra. The resulting FY-XAS spectra are compared with the normal XAS spectra corresponding to the absorption cross section and significant deviations between the two spectra are found. This implies that the assumption that the FY-XAS spectrum identifies with the XAS spectrum is disproved. Especially for the early transition metal systems the differences between the FY-XAS and XAS are large, due to the opening of inelastic decay channels from selected x-ray absorption final states. The theoretical calculations show that the difference between FY detection and XAS is largest for the detection in depolarized geometry. The calculations are compared with experimental spectra for oxides and coordination compounds for Fe2+, Co2+ and Ni2+ systems. The implications for the sum rules in XAS and magnetic circular dichroism experiments are discussed