We leverage s-orbital non-resonant inelastic X-ray scattering (s-NIXS) to perform orbital imaging on three bulk rare-earth nickelates spanning a range of formal nickel valence (3d electron filling) from Ni^3+ (3d^7) to Ni^1+ (3d^9). Our results directly reveal the ground states of these compounds all with minimal theoretical input. In particular, we demonstrate the low-spin orbital configuration of trivalent LaNiO_3, the d_x^2-y^2 configuration of monovalent LaNiO_2, and resolve the effective e_g crystal field splitting in the distorted octahedral environment of divalent La_2NiO_4. This work illustrates the potential of s-NIXS to study the ground state and excited states of strongly correlated materials without needing complex theoretical analysis of spectroscopic data.

UCd_{11} is an antiferromagnetic uranium intermetallic compound (T_{N}=5.3K) with enhanced electron mass and uranium-uranium spacings nearly twice the Hill limit, suggesting a weakly hybridized 5f electronic character. Various x-ray spectroscopy techniques indicate that uranium in UCd_{11} adopts the formal U^{3+}5f^{3} configuration, while core-level photoemission spectroscopy (PES) data of UCd_{11} reveal only a weak satellite feature, typically interpreted as a signature of itinerancy. In this work, we present density functional theory (DFT) combined with dynamical mean-field theory (DMFT) calculations of UCd_{11}, using material-specific parameters tuned to reproduce valence-band PES spectra at different photon energies, thereby exploiting the energy dependence of photoionization cross sections. Our results demonstrate that UCd_{11} is a highly localized uranium 5f^{3} system. Furthermore, core-level spectra obtained from a DFT+DMFT Anderson impurity model reveal that, contrary to common assumptions, the presence or absence of satellite structures is not a reliable indicator of strong correlations or itinerant 5f behavior.

We carried out nonresonant inelastic x-ray scattering measurements at the V L_{1} edge to probe the local V 3d charge density in VO_{2} across the metal-insulator transition (MIT). Without relying on spectral calculations, we were able to integrate directly from the orientational dependence an image of the occupied 3d shell and extract quantitatively the orbital occupations of the V ions. We found that in the low-temperature insulating M_{1} phase the V is highly polarized toward the σ configuration. The orbital occupation undergoes significant redistribution upon transition into the metallic R phase, leading to a more isotropic charge density. The massiveness of the orbital switching promotes the scenario in which electron correlations and lattice degrees of freedom are strongly coupled, so that the contribution of the lattice and the electronic part to the energetics must be treated on equal footing to model the MIT quantitatively.

Electron correlation in solids has a major impact on material properties. However, it has been studied mainly by theory, with very limited direct experimental investigations. Here, we demonstrate that dynamic electron correlation function can be experimentally measured using inelastic x-ray scattering on polycrystalline beryllium. The data are expressed as the energy-resolved dynamic pair-distribution function. Our results confirm the size of the exchange-correlation hole as ∼2  Å, consistent with theoretical expectations. However, at the plasmon energy of ∼21  eV, the exchange-correlation hole is extended up to 4-5 Å, suggesting a unique influence of the dynamic plasmon state.

The total energy resolution (ΔEtot) of a valence-band resonant inelastic X-ray scattering (VB-RIXS) instrument serves as an important point of reference in an otherwise complex field. Since VB-RIXS is a flux-limited technique, a pragmatic approach to reducing ΔEtot is often required—the specifications of a spectrometer should be matched with a comparable incident bandwidth (ΔEi) and the source size contribution (focal point) should be negligible. Although it advocates for a good efficiency, this approach is in many places already limited by count-rates. Here we follow a recent trend emerging in soft X-ray VB-RIXS and look at the performance of our tender X-ray Rowland spectrometer (Gretarsson et al., 2020) when being exposed to a source with a large linear dispersion (higher flux). Detailed ray tracing work, performed at the U M5-edge (3551 eV), finds that the intrinsic resolution of the Rowland spectrometer (ΔEa) can be obtained if the linear dispersion of the source matches the spectrometer's, but opposite in sign—here ΔEi does not matter. This finding is supported by experimental data where ΔEtot = 48 meV (ΔEa = 44 meV) was recently achieved. Furthermore, we demonstrate that the dispersion rate can be tuned, ensuring the method's applicability to other atomic edges.

All-solid-state sodium metal batteries are an attractive alternative to Li-ion batteries due to their high energy density, improved safety, and the lower price of sodium. A major bottleneck is the development of suitable solid electrolytes with sufficient ionic conductivity and electrochemical stability. Here, we report new all-solid-state sodium metal batteries based on sodium closo-dodecahydridoborate (Na2B12H12) electrolyte and both Prussian white (Na2MnFe(CN)6) and TiS2 cathode active materials. Although the pristine Na2B12H12 has a low ionic conductivity at room temperature, the conductivity increases by more than three orders of magnitude upon nanocomposite formation with mesoporous SiO2via high-energy ball milling. The high ionic conductivity (5 & times; 10-4 S cm-1, 30 degrees C) and oxidative stability (3.9 V vs. Na+/Na) of Na2B12H12/SiO2 enable room-temperature battery operation with impressive capacity retention. Using a variety of techniques, including X-ray Raman scattering and electron energy loss spectroscopy, we show the presence of an interphase formed by an interface reaction between Na2B12H12 and SiO2. The observed interface effects are highly influenced by the morphology of the oxidic framework and the preparation conditions of the nanocomposite.

UCd11 is an antiferromagnetic uranium intermetallic compound (TN = 5.3 K) with enhanced electron mass and uranium-uranium spacings nearly twice the Hill limit, suggesting a weakly hybridized 5 f electronic character. Various x-ray spectroscopy techniques indicate that uranium in UCd11 adopts the formal U3+ 5 f3 configuration, while core-level photoemission spectroscopy (PES) data of UCd11 reveal only a weak satellite feature, typically interpreted as a signature of itinerancy. In this work, we present density functional theory (DFT) combined with dynamical mean-field theory (DMFT) calculations of UCd11, using material-specific parameters tuned to reproduce valence-band PES spectra at different photon energies, thereby exploiting the energy dependence of photoionization cross sections. Our results demonstrate that UCd11 is a highly localized uranium 5 f3 system. Furthermore, core-level spectra obtained from a DFT + DMFT Anderson impurity model reveal that, contrary to common assumptions, the presence or absence of satellite structures is not a reliable indicator of strong correlations or itinerant 5 f behavior.

The mechanism of a pressure-induced quantum critical point in the heavy fermion ferromagnet CeRh6Ge4 has attracted interest, as ferromagnetic quantum criticality in a clean itinerant Ce compound is typically avoided. The localized versus itinerant character of the 4 f electrons is a key aspect for understanding this behavior. We investigated the electronic structure of the 4 f shell in CeRh6Ge4 using core-level photoelectron and x-ray absorption spectroscopy, demonstrating the hybridization of Ce 4 f with the conduction electrons. Linearly polarized x-ray absorption reveals a temperature-dependent linear dichroism consistent with the crystalelectric-field sequence as inferred from the static susceptibility. This dichroism cannot be described by an ionic full-multiplet model alone, but is reproduced by including the Kondo effect within a single-impurity Anderson model in the noncrossing approximation. The Kondo effect mixes higher-lying crystal-field states into a resulting multiorbital ground state with 4 f occupancy, nf 0.9. Deviations at low temperatures between the measured linear dichroism and calculated dichroism suggest an orbital-dependent Kondo effect. A scenario in which there is a multiorbital ground state and orbital-dependent Kondo hybridization should be a starting point for a model of pressure-induced criticality in CeRh6Ge4.

The mechanism behind superconductivity suppression induced by Pr substitutions in YBa2Cu3O7-δ (YBCO) has been a mystery since its discovery: in spite of being isovalent to Y3+ with a small magnetic moment, it is the only rare-earth element that has a dramatic impact on YBCO's superconducting properties. Using angle-resolved photoemission spectroscopy (ARPES) and DFT+[Formula: see text] calculations, we uncover how Pr substitution modifies the low-energy electronic structure of YBCO. Contrary to the prevailing Fehrenbacher-Rice (FR) and Liechtenstein-Mazin (LM) models, the low-energy electronic structure contains no signature of any f-electron hybridization or additional f-state Fermi surface sheets. Yet, strong electron doping is observed primarily on the antibonding Fermi surface. Meanwhile, we reveal major electronic structure modifications to Cu-derived states with increasing Pr substitution: a pronounced CuO2 bilayer decoupling and enhanced hopping along the CuO chain, implying indirect electron-release pathways beyond simple 4f state ionization. Our results challenge the long-standing FR/LM mechanism, and establish Pr substituted YBCO as a potential platform for exploring correlation-driven phenomena in coupled 1D-2D systems.

Solid-state electrolytes play a key role in the development of safe and high-capacity all-solid-state batteries. Complex hydrides such as Li2B12H12 are attractive as solid electrolytes due to their low weight and good electrochemical stability, but suffer from low conductivities at room temperature. Herein, we report a three-order-magnitude increase in the ionic conductivity of Li2B12H12 upon nanocomposite formation with ZrO2 via mechanochemical treatment, reaching 2.9 × 10-4 S cm-1 at 30 °C. Results from infrared spectroscopy, X-ray Raman scattering and electron microscopy coupled with electron energy loss spectroscopy suggest that the increased ionic conductivity is due to strong interfacial interaction/reaction between Li2B12H12 and ZrO2. This leads to a highly defective interphase region where the Li, B, Zr, and O chemical environments are distinctively different from the bulk Li2B12H12 and ZrO2. The improved ionic conductivity of the nanocomposite compared to the pristine material enabled the realization of all-solid-state batteries with a Li metal anode and both TiS2 and LiFePO4 cathodes. We demonstrate the suitability of the nanocomposite at various charging rates up to C/2 (0.34 mA cm-2) for over 170 cycles at 40-60 °C (Li|Li2B12H12/ZrO2|TiS2).

Understanding the electronic structure of actinide materials is crucial for both fundamental research and nuclear applications. The partially filled 5f shells exhibit complex behavior due to strong correlations and ligand hybridization, requiring advanced spectroscopic techniques. Here, we report on the development and application of high-resolution valence band resonant inelastic x-ray spectroscopy (VB-RIXS) experiments at the uranium M_{4,5} edges (3551 and 3725 eV). We present data of UO_{2}, a well-established model actinide compound. VB-RIXS is particularly well suited for probing the 5f-shell electronic structure, as it probes, in contrast to core-to-core RIXS, excitations without leaving a high-energy core hole in the final state. In VB-RIXS, we achieve energy resolutions of 50 meV (M_{5}) and 90 meV (M_{4}), enabling the resolution of multiplet excitations and crystal-field effects, as well as charge-transfer and fluorescencelike features with unprecedented clarity. As such, high-resolution VB-RIXS offers direct insights into both low-energy, near-ground-state properties and high-energy hybridization and covalency effects. Our results demonstrate the power of VB-RIXS as a versatile and powerful tool for probing the strongly correlated electronic structure of actinide materials, providing essential input for quantitative modeling and the validation of theoretical concepts.

Resonant inelastic x-ray scattering (RIXS) is a powerful tool for probing the ground-state electronic configurations of actinide materials. However, in certain metallic uranium compounds, RIXS fails to detect excitations from the ground-state multiplet. This absence is attributed to strong hybridization between uranium 5 f electrons and conduction electrons. In the present study, we extend RIXS investigations to a 16 mu g sample of the metallic compound AmFe2, marking an experiment at the M4,5 edges of a transuranium material. Americium in AmFe2 adopts a trivalent (Am3+) state with a 5 f 6 electronic configuration having a J = 0 ground state, with the 5 f states located approximately 3 eV below EF. The RIXS spectra exhibit well-resolved features that are in good agreement with theory. The localized nature of the 5 f electrons in AmFe2 permits the observation of multiplet excitations despite its metallic character. These observations extend our understanding of RIXS in actinide systems.

We investigate the nature of the 5f electrons in the unconventional odd-parity superconductor UTe_{2}, focusing on the degree of covalency, localization versus itinerancy, and dominant electronic configuration. This is achieved using density functional theory (DFT) in combination with dynamical mean-field theory (DMFT) calculations. A key aspect of our approach is the material-specific tuning of the double-counting correction parameter, μ_{dc}, within the DFT+DMFT part. This tuning is guided by the energy dependence of photoionization cross sections in valence band photoelectron spectroscopy. The reliability of the parameters is confirmed by the accurate reproduction of the angle-resolved valence-band photoemission spectra and the U4f core-level data. The DFT+DMFT model reveals that in UTe_{2} U5f^{n} configurations with n=1–4 contribute to the ground state, with the 5f^{2} configuration being most prevalent and an average 5f shell filling close to 2.5. The model further suggests that the 5f electrons form narrow bands and that charge fluctuations due to degeneracy play a role in addition to coherent valence dynamics arising from hybridization with the conduction bath. Additionally, the significance of the U6d states in UTe_{2} is discussed.

Understanding the electronic structure of actinide materials is crucial for both fundamental research and nuclear applications. The partially filled 5f shells exhibit complex behavior due to strong correlations and ligand hybridization, requiring advanced spectroscopic techniques. Here, we report on the development and application of high-resolution valence-band resonant inelastic x-ray spectroscopy (VB-RIXS) experiments at the uranium M_4,5 edges (3551 and 3725 eV). We present data of UO_2, a well-established model actinide compound. VB-RIXS is particularly well suited for probing the 5f-shell electronic structure, as it probes, in contrast to core-to-core RIXS, excitations without leaving a high-energy core hole in the final state. In VB-RIXS, we achieve energy resolutions of 50 meV (M_5) and 90 meV (M_4), enabling the resolution of multiplet excitations and crystal-field effects, as well as charge-transfer and fluorescence-like features with unprecedented clarity. As such, high resolution VB-RIXS offers direct insights into both low-energy, near ground-state properties and high-energy hybridization and covalency effects. Our results demonstrate the power of VB-RIXS as a versatile and powerful tool for probing the strongly correlated electronic structure of actinide materials, providing essential input for quantitative modeling and the validation of theoretical concepts.

Solid-state sodium ion conductors are crucial for the next generation of all-solid-state sodium batteries with high capacity, low cost, and improved safety. Sodium closo-carbadodecaborate (NaCB11 H12 ) is an attractive Na-ion conductor owing to its high thermal, electrochemical, and interfacial stability. Mechanical milling has recently been shown to increase conductivity by five orders of magnitude at room temperature, making it appealing for application in all-solid-state sodium batteries. Intriguingly, milling longer than 2 h led to a significant decrease in conductivity. In this study, X-ray Raman scattering (XRS) spectroscopy is used to probe the origin of the anomalous impact of mechanical treatment on the ionic conductivity of NaCB11 H12 . The B, C, and Na K-edge XRS spectra are successfully measured for the first time, and ab initio calculations are employed to interpret the results. The experimental and computational results reveal that the decrease in ionic conductivity upon prolonged milling is due to the increased proximity of Na to the CB11 H12 cage, caused by severe distortion of the long-range structure. Overall, this work demonstrates how the XRS technique, allowing investigation of low Z elements such as C and B in the bulk, can be used to acquire valuable information on the electronic structure of solid electrolytes and battery materials in general.

We investigate the topological superconductor candidate UTe$_2$ using high-resolution valence-band resonant inelastic x-ray scattering at the U $M_{4,5}$-edges. We observe atomic-like low-energy excitations that support the correlated nature of this unconventional superconductor. These excitations originate from the U $5f^2$ configuration, which is unexpected since the short Te2-Te2 distances exclude Te2 being 2-. By utilizing the photoionization cross-section dependence of the photoemission spectra in combination with band structure calculations, we infer that the stabilization of the U $5f^2$ configuration is due to the U $6d$ bonding states in the U-dimers acting as a charge reservoir. Our results emphasize that the description of the physical properties should commence with a $5f^2$ $ansatz$.

A procedure for quantifying the U 5 f electronic covalency and degree of localization in U intermetallic compounds is presented. To this end, bulk sensitive hard and soft x-ray photoelectron spectroscopy were utilized in combination with density-functional theory (DFT) plus dynamical mean-field theory (DMFT) calculations. The energy dependence of the photoionization cross sections allows the disentanglement of the U 5 f contribution to the valence band from the various other atomic subshells so the computational parameters in the DFT + DMFT can be reliably determined. Applying this method to UGa2 and UB2 as model compounds from opposite ends of the (de)localization range, we have achieved excellent simulations of the valence band and core-level spectra. The width in the distribution of atomic U 5 f configurations contributing to the ground state, as obtained from the calculations, quantifies the correlated nature and degree of localization of the U 5 f. The findings permit answering the longstanding question why different spectroscopic techniques give seemingly different numbers for the U 5 f valence in intermetallic U compounds.

CeRh2As2 is a new multiphase superconductor with strong suggestions for an additional itinerant multipolar ordered phase. The modeling of the low -temperature properties of this heavy-fermion compound requires a quartet Ce3 thorn crystal -field ground state. Here, we provide the evidence for the formation of such a quartet state using x-ray spectroscopy. Core -level photoelectron and x-ray absorption spectroscopy confirm the presence of Kondo hybridization in CeRh2As2. The temperature dependence of the linear dichroism unambiguously reveals the impact of Kondo physics for coupling the Kramer's doublets into an effective quasiquartet. Nonresonant inelastic x-ray scattering data find that the j Gamma-7i state with its lobes along the 110 direction of the tetragonal structure (xy orientation) contributes most to the multiorbital ground state of CeRh2As2.

Solid-state electrolytes (SSEs) with high ionic conductivities are crucial for safer and high-capacity batteries. Interface effects in nanocomposites of SSEs and insulators can lead to profound increases in conductivity. Understanding the composition of the interface is crucial for tuning the conductivity of composite solid electrolytes. Herein, X-ray Raman Scattering (XRS) spectroscopy is used for the first time to unravel the nature of the interface effects responsible for conductivity enhancements in nanocomposites of complex hydride-based electrolytes (LiBH4, NaBH4, and NaNH2) and oxides. XRS probe of the Li, Na, and B local environments reveals that the interface consists of highly distorted/defected and structurally distinct phase(s) compared to the original compounds. Interestingly, nanocomposites with higher concentrations of the interface compounds exhibit higher conductivities. Clear differences are observed in the interface composition of SiO2- and Al2O3-based nanocomposites, attributed to differences in the reactivity of their surface groups. These results demonstrate that interfacial reactions play a dominant role in conductivity enhancement in composite solid electrolytes. This work showcases the potential of XRS in investigating interface interactions, providing valuable insights into the often complex ion conductor/insulator interfaces, especially for systems containing light elements such as Li, B, and Na present in most SSEs and batteries. X-ray Raman spectroscopy is employed to unravel the nature of the interface in nanocomposites of complex hydrides and metal oxides, and thereby the origin of the profound increase in ionic conductivity in these nanocomposite solid electrolytes. The results reveal that surface reactions at ion conductor-insulator interfaces play a dominant role in interfacial ion transport.image

The application of pressure as well as the successive substitution of Ru with Fe in the hidden order (HO) compound URu2Si2 leads to the formation of the large-moment antiferromagnetic phase. Here, we investigate the substitution series URu2-xFexSi2 from x = 0.0 to 2.0 by U 4 f core-level photoelectron spectroscopy and observe nonmonotonic changes in the spectra. The initial increase and subsequent decrease in the spectral weight of the 4 f core-level satellite with increasing x stands for a nonmonotonic 5 f filling across the substitution series. The competition of chemical pressure and increase in the density of states at the Fermi energy, both due to substitution of Ru with Fe, can explain such behavior. An extended Doniach phase diagram including the x dependence of the density of states is proposed. Also in URu2-xFexSi2 the ground state is a singlet or quasidoublet state consisting of two singlets. Hence, the formation of magnetic order in the URu2-xFexSi2 substitution series must be explained within a singlet magnetism model.