Logo: to the web site of Uppsala University

Uranium dioxide (UO₂) is a complex material with significant relevance to nuclear energy, materials science, and fundamental research. Understanding its high-temperature behavior is crucial for developing new uranium-based materials and improving nuclear fuel efficiency in nuclear reactors. Here we study the evolution of uranium state during the oxidation of UO₂ in air at temperatures up to 550 °C using the in situ X-ray absorption spectroscopy in high energy resolution fluorescence detection mode at the U M₄ edge, combined with electronic structure calculations. Our data reveal a complex sequence of events occurring over minutes and hours at elevated temperatures, including changes in the electronic and local structure, 5f electron occupancy, the formation of U cuboctahedral clusters, and the creation of U₄O₉ and U₃O₇ mixed U oxide phases. These findings highlight the fundamental role of clustering processes and pentavalent uranium in both the oxidation process and the stabilization of uranium materials.

The electronic structure and the band gap behavior of CH₃NH₃Pb(I_1−xBr_x)₃ for x = 0.25, 0.33, 0.50, 0.67, 0.75, 1.00 were studied using the full-relativistic density-functional-theory calculations. A combination of the parameter-free Armiento–Kümmel generalized gradient approximation exchange functional with the nonseparable gradient approximation Minnesota correlation functional was employed. The calculated band gap sizes for the CH₃NH₃Pb(I_1−xBr_x)₃ series were found to be similar to the experimentally measured values. While the change of the optimized lattice parameter with an increasing Br content can be described by a linear fit, the calculated band gap variation exhibits rather a quadratic-like behavior over the x region of the cubic crystal structure. While the experimental reports are divided on whether the bowing parameter value is being very small or significant, our calculated results support the latter case.

The employment of the parameter-free Armiento-Kummel generalized gradient approximation (AK13-GGA) exchange functional was examined as a means of the band gap prediction for hybrid metal halide perovskites (HaPs) or systems with strong spin-orbit coupling in the full-relativistic density-functional theory (DFT) calculations. A combination of AK13 with the nonseparable gradient approximation Minnesota correlation functional (GAM) was established as an approach allowing for the efficient band gap estimation with accuracy similar to the GW approximation method but at the computational costs of conventional DFT. This was further supported by results of the AK13/GAM calculations performed for various HaPs. The described approach creates an opportunity for the effective assessment of the electronic structure of large, complex, doped, or defective HaPs and modeling of advanced materials.

Chalcogenide perovskites exhibit optoelectronic properties that position them as potential materials in the field of photovoltaics. We report a detailed investigation into the electronic structure and chemical properties of polycrystalline BaZrS3 perovskite powder by X-ray photoelectron spectroscopy, complemented by an analysis of its long- and short-range geometric structures using X-ray diffraction and X-ray absorption spectroscopy. The results obtained for the powdered BaZrS3 are compared to similar measurements on a sputtered polycrystalline BaZrS3 thin film prepared through rapid thermal processing. While bulk characterization confirms the good quality of the powder, depth-profiling achieved by photoelectron spectroscopy utilizing Al K-alpha (1.487 keV) and Ga K-alpha (9.25 keV) radiations shows that, regardless of the fabrication method, the oxidation effects extend beyond 10 nm from the sample surface, with zirconium oxides specifically distributing deeper than the oxidized sulfur species. A hard X-ray photoelectron spectroscopy study on the powder and thin film detects signals with minimal contamination contributions and allows for the determination of the valence band maximum position with respect to the Fermi level. Based on these measurements, we establish a correlation between the experimental valence band spectra and the theoretical density of states derived from density functional theory calculations, thereby discerning the orbital constituents involved. Our analysis provides an improved understanding of the electronic structure of BaZrS3 developed through different synthesis protocols by linking it to material geometry, surface chemistry, and the nature of doping. This methodology can thus be adapted for describing electronic structures of chalcogenide perovskite semiconductors in general, a knowledge that is significant for interface engineering and, consequently, for device integration.

The Am N_4,5 (4d_3/2 and 4d_5/2) and Am O_4,5 (5d_3/2 and 5d_5/2) x-ray absorption spectroscopy (XAS) of americium sesquioxide (Am₂O₃) and americium dioxide (AmO₂) has been evaluated with FEFF, a Green's function–based, multiple scattering code. Taking guidance from the intermediate coupling model (ICM), applicable to local and nonmagnetized samples, it is possible to completely reconstruct the experimental results for the N_4,5 spectra, including the observed differences between the Am₂O₃ and the AmO₂ cases. Although complicated by a more asymmetric line shape and difficult background variations, the FEFF analysis confirms the absence of core hole angular momentum coupling in Am O_4,5 spectroscopy.

The electronic structure and the chemical state in Am binary oxides and Am-doped UO2 were studied by means of X-ray absorption spectroscopy at shallow Am core (4d and 5d) edges. In particular, the Am 5f states were probed and the nature of their bonding to the oxygen states was analyzed. The interpretation of the experimental data was supported by the Anderson impurity model (AIM) calculations which took into account the full multiplet structure due to the interaction between 5f electrons as well as the interaction with the core hole. The sensitivity of the branching ratio of the Am 4d(3/2) and 4d(5/2)X-ray absorption lines to the chemical state of Am was shown using Am binary oxides as reference systems. The observed ratio for Am-doped UO2 suggests that at least at low Am concentrations, americium is in the Am(III) state in the UO2 lattice. To confirm the validity of the applied AIM approach, the analysis of the Am 4fX-ray photoelectron spectra of AmO2 and Am2O3 was also performed which revealed a good agreement between experiment and calculations. As a whole, AmO2 can be classified as the charge-transfer compound with the 5f occupancy (n(f)) equal to 5.73 electrons, while Am2O3 is rather a Mott-Hubbard system with n(f) = 6.05.

The electronic structure of UCx (x = 0.9, 1.0, 1.1, 2.0) was studied by means of x-ray absorption spectroscopy (XAS) at the CK 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 5f - 5f Coulomb interaction U and spin-orbit coupling (DFT+U+SOC) were also performed for UCand UC2. While the U L-3 HERFD-XAS spectra of the studied samples reveal little difference, the U M-4 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 C2p-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 UC2 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 UC2, thus affecting the thermodynamic properties. Both the x-ray spectroscopic data (in particular, the CK 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.

The Am 5d-5f resonant inelastic x-ray scattering (RIXS) data of americium sesquioxide were measured at incident photon energies throughout the Am O4,5 edges. The experiment was supported by calculations using several model approaches. While the experimental Am O4,5 x-ray absorption spectrum of Am2O3 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-5 f 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 Am2O3 is singlet P1. An appearance of the low-intense charge-transfer satellite in the Am 5d-5 f RIXS spectra at an energy loss of similar to 5.5 eV suggests weak Am 5 f-O 2p hybridization which is in agreement with AIM estimations of the 5 f occupancy from spectroscopic data in Am2O3 as being 6.05 electrons.

The prototypical chalcogenide perovskite BaZrS3, characterized by its direct band gap, exceptionally strong light-harvesting ability, and good carrier transport properties, provides fundamental prerequisites for a promising photovoltaic material. This inspired the synthesis of BaZrS3 in the form of thin films, using sputtering and rapid thermal processing, aimed at device fabrication for future optoelectronic applications. Using a combination of short- and long-range structural information from X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD), we have elucidated how, starting from a random network of Ba, Zr, and S atoms, thermal treatment induces crystallization and growth of BaZrS3 and explained its impact on the observed photoluminescence (PL) properties. We also provide a description of the electronic structure and substantiate the surface material chemistry using a combination of depth-dependent photoelectron spectroscopy (PES) using hard X-ray (HAXPES) and traditional Al K alpha radiation. From the knowledge of the optical band gap of BaZrS3 thin films, synthesized at an optimal temperature of 900 C-degrees, and our estimation of the valence band edge position with respect to the Fermi level, one may conclude that these semiconductor films are intrinsic in nature with a slight n-type character. A detailed understanding of the growth mechanism and electronic structure of BaZrS3 thin films helps pave the way toward their utilization in photovoltaic applications.

We report on a combined experimental and theoretical study on CrI3 single crystals by employing the polarization dependence of resonant inelastic x-ray scattering (RIXS). Our investigations reveal multiple Cr 3d orbital splitting (dd excitations) as well as magnetic dichroism (MD) in the RIXS spectra. The dd excitation energies are similar on the two sides of the ferromagnetic transition temperature, T-C similar to 61 K, although MD in RIXS is predominant at 0.4 T magnetic field below TC. This demonstrates that the ferromagnetic superexchange interaction that is responsible for the interatomic exchange field is vanishingly small compared with the local exchange field that comes from exchange and correlation interaction among the interacting Cr 3d orbitals. The recorded RIXS spectra reported here reveal clearly resolved Cr 3d intraorbital dd excitations that represent transitions between electronic levels that are heavily influenced by dynamic correlations and multiconfiguration effects. Our calculations taking into account the Cr 3d hybridization with the ligand valence states and the full multiplet structure due to intra-atomic and crystal field interactions in Oh and D3d symmetry clearly reproduced the dichroic trend in experimental RIXS spectra.