Johan Vegelius

High-resolution X-ray absorption spectroscopy as a probe of crystal-field and covalency effects in actinide compounds

Significance This work is to our knowledge the first illustration of the ability of the high-energy resolution fluorescence-detection X-ray absorption spectroscopy (HERFD-XAS) technique to directly probe the crystal-field splitting in the 5f shell of actinides. This is a breakthrough for actinide science, allowing for the easy extraction of desired information from the spectroscopic method, which is easy to interpret and to calculate. Furthermore, the HERFD-XAS technique allows us to resolve the charge-transfer satellites in actinide spectra that were hidden before, thus enhancing the sensitivity to the covalent character of the chemical bonding. Using ThO2 as an example, we show that other common techniques underestimate the crystal-field strength and that ThO2 is not an anionic compound as previously believed. Applying the high-energy resolution fluorescence-detection (HERFD) mode of X-ray absorption spectroscopy (XAS), we were able to probe, for the first time to our knowledge, the crystalline electric field (CEF) splittings of the 5f shell directly in the HERFD-XAS spectra of actinides. Using ThO2 as an example, data measured at the Th 3d edge were interpreted within the framework of the Anderson impurity model. Because the charge-transfer satellites were also resolved in the HERFD-XAS spectra, the analysis of these satellites revealed that ThO2 is not an ionic compound as previously believed. The Th 6d occupancy in the ground state was estimated to be twice that of the Th 5f states. We demonstrate that HERFD-XAS allows for characterization of the CEF interaction and degree of covalency in the ground state of actinide compounds as it is extensively done for 3d transition metal systems.

Sensitivity to actinide doping of uranium compounds by resonant inelastic X-ray scattering at uranium L3 edge.

Valence-to-core resonant inelastic X-ray scattering (RIXS) and high energy resolution fluorescence detection (HERFD) X-ray absorption measurements were performed at the U L3 edges of UO2 and UO2(NO3)2(H2O)6. The results are compared with model calculations based on the local-density-approximation formalism, taking into account Coulomb interaction U (LDA + U). We show that despite strong 5f-5f electronic correlations in the studied systems and the use of core-level excitations in the intermediate stage of the spectroscopic process, the RIXS technique probes a convolution of the single-particle densities of states in the valence and conduction bands. For UO2, the detected crystal-field splitting between the U 6d eg and t2g orbitals from the RIXS spectra (∼3.5 eV) is larger than that previously derived from optical spectroscopy. Furthermore, by using an example of the U0.75Pu0.25O2 mixed oxide, we show that the RIXS technique at the U L3 edges is sensitive to the substitution of U with other actinide, in contrast to conventional X-ray absorption methods. That is, due to changes in the occupied part rather than in the unoccupied part of the U 6d states caused by the substitution.

5 f -shell correlation effects in dioxides of light actinides studied by O 1s x-ray absorption and emission spectroscopies and first-principles calculations.

Soft x-ray emission and absorption spectroscopic data are reported for the O 1s region of a single crystal of UO2, a polycrystalline NpO2 sample, and a single crystal of PuO2. The experimental data are interpreted using first-principles correlated-electron calculations within the framework of the density functional theory with added Coulomb U interaction (DFT+U). A detailed analysis regarding the origin of different structures in the x-ray emission and x-ray absorption spectra is given and the effect of varying the intra-atomic Coulomb interaction-U for the 5 f electrons is investigated. Our data indicate that O 1s x-ray absorption and emission spectroscopies can, in combination with DFT+U calculations, successfully be used to study 5 f -shell Coulomb correlation effects in dioxides of light actinides. The values for the Coulomb U parameter in these dioxides are derived to be in the range of 4-5 eV.

Cu Kβ2,5 X-ray emission spectroscopy as a tool for characterization of monovalent copper compounds

Cu Kβ2,5 X-ray emission and resonant inelastic X-ray scattering measurements were performed on monovalent and divalent copper compounds. The data were compared with the results of local-density-approximation calculations. The methods were found to be efficient tools for studies of Cu 4p states in the valence band and for distinguishing between different monovalent copper compounds. This is of particular importance for the debate concerning copper corrosion in oxygen-free water.

Local Symmetry Effects in Actinide 4f X-ray Absorption in Oxides

A systematic X-ray absorption study at actinide N6,7 (4f → 6d transitions) edges was performed for light-actinide oxides including data obtained for the first time for NpO2, PuO2, and UO3. The measurements were supported by ab initio calculations based on local-density-approximation with added 5f-5f Coulomb interaction (LDA+U). Improved energy resolution compared to common experiments at actinide L(2,3) (2p → 6d transitions) edges allowed us to resolve the major structures of the unoccupied 6d density of states (DOS) and estimate the crystal-field splittings in the 6d shell directly from the spectra of light-actinide dioxides. The measurements demonstrated an enhanced sensitivity of the N(6,7) spectral shape to changes in the compound crystal structure. For nonstoichiometric NpO(2-x), the filling of the entire band gap with Np 6d states was observed thus supporting a phase coexistence of Np metal and stoichiometric NpO2 which is in agreement with the tentative Np-O phase diagram.

Temperature dependence of the electrical resistivity and electronic structure of amorphous Fe100−xZrx films and multilayers

The electrical resistivity of amorphous Fe(100-x)Zr(x) metal alloy films and multilayers has been investigated in a wide temperature and composition range. The overall behavior of the resistivity is consistent with bulk measurements, exhibiting prominent semiconductor-like changes at low temperatures. The transition from positive (metallic) to negative temperature coefficient of resistivity behavior is accompanied by minute changes in magnetoresistance and we can therefore rule out magnetic phase changes as being the cause for the observed changes in the resistivity. Using x-ray absorption and emission spectroscopies we are able to probe the unoccupied and occupied electronic densities of states. The corresponding spectra are found to significantly overlap, as expected for a metallic-like electronic structure and the absence of a band gap. Besides a broadening of the x-ray emission lines expected from an amorphous material, remarkably small differences are observed in the electronic structures when changing the amount of Zr. The resistivity data were modeled and agreement with the Mott variable range hopping model was found, indicating localized electronic states due the disordered structure of the Fe(100-x)Zr(x) alloys.

Atomic and electronic structure of amorphous Al–Zr alloy films

Amorphous Al(73)Zr(27) alloy film, grown and then subjected to heat treatments at 400 and 700 °C, was studied using a combination of x-ray diffraction and soft x-ray spectroscopic techniques. The Al L(2,3) and Al K x-ray absorption spectroscopy (XAS) and Al L(2,3) x-ray emission spectroscopy (XES) used allowed probing the unoccupied and occupied Al 3s, d states and unoccupied Al 3p states in the sample studied. An irreversible transition from amorphous alloy to a mixture of polycrystalline alloy and amorphous alloy, and then to an amorphous oxide phase was observed. After the annealing at 400 °C the Al L(2,3) spectra obtained by XAS could be explained as sums of spectra from amorphous Al(73)Zr(27) alloy and (poly)crystalline Al. This indicates that the sample consists of a mixture of Al-rich crystalline and Zr-enhanced amorphous alloys, as compared to the stoichiometry of the as-deposited Al(73)Zr(27) sample, and that the electronic wavefunctions in the crystalline and amorphous regions can be considered to be confined within the respective regions. The relative amounts of Al atoms were found to be around 1:3 in the crystalline and amorphous phases, respectively, as deduced from the analysis of changes in the electronic structure using Al L(2,3) XAS data. The interpretation was confirmed by the Al K XAS and Al L(2,3) XES. Upon further annealing at 700 °C the polycrystalline phase transformed into amorphous oxide, while the amorphous alloy phase underwent gradual oxidation. The important finding was that the greater part of the sample remained in the amorphous state throughout the temperature regimes described.