Frank M. F. de Groot

The Ti L 2 , 3 − edge x-ray absorption spectra of various titanium oxides, including perovskite SrTi O 3 , rutile and anatase Ti O 2 , and ilmenite CoTi O 3 , are analyzed with particular attention to the energy range beyond the near-edge/charge-transfer region and up to the oxygen K edge. We found that the spectral structure varies among materials and can be described using density functional theory without including the multielectron effects indispensable for describing near-edge and charge-transfer structures. We term the structures in this energy range the middle-range-energy structure and show that the distinct features are determined by the local atomic geometry around the target Ti atoms and, essentially, by the partial density of states of those atoms. Here, we demonstrate how to extract information from middle-range-energy structures by comparing them with calculations, and we propose this energy-range analysis as a new method for studying local atomic geometry, free of parameters describing multielectron effects.

Understanding the intricate interplay between electronic and lattice dynamics is indispensable for controlling photoinduced functionality in correlated oxides. Here, we investigate the ultrafast relaxation pathways in textured LaFeO3 using femtosecond extreme-ultraviolet (XUV) absorption spectroscopy at the iron 3p edge. These nanosheets facilitate oriented perovskite growth on the amorphous Si3N4 membrane, giving rise to local epitaxy while preserving XUV transparency. By combining table-top time-resolved measurements with crystal-field multiplet calculations, we track the evolution from rapid ligand-to-metal charge transfer (LMCT) excitation to a lattice-stabilized polaronic state. The early-time response is dominated by the population of a mixed charge-transfer state with pronounced t2g3eg3 (5E) character, which indicates the initial electronic redistribution that follows photoexcitation. On subpicosecond time scales, the emergence of lattice-assisted carrier localization leads to partial charge localization and a lowering of the symmetry of Fe3+ sites. These changes suggest the formation of a polaronic state. At later delays, the transient spectra are governed by distorted Fe3+ configurations with residual charge-transfer character, without evidence for long-lived Fe2+ formation. Comparison with α-Fe2O3 reveals distinct material dependence in both charge-transfer relaxation and polaron-formation dynamics, demonstrating the roles of lattice topology and iron–oxygen covalency.

Tantalum dioxide (TaO2) is a metastable tantalum compound. Here, we report the epitaxial stabilization of TaO2 on Al2O3 (11̄02) (r-plane sapphire) substrates using suboxide molecular-beam epitaxy and thermal laser epitaxy, demonstrating single-oriented, monodomain growth of anisotropically strained thin films. Microstructural investigation is performed using synchrotron x-ray diffraction and scanning transmission electron microscopy. The tetravalent oxidation state of tantalum is confirmed using x-ray absorption and photoemission spectroscopy as well as electron energy-loss spectroscopy. Optical properties are investigated via spectroscopic ellipsometry and reveal a 0.3 eV Mott gap of the tantalum 5d electrons. Density-functional theory and group theoretical arguments are used to evaluate the limited stability of the rutile phase and reveal the potential to unlock a hidden metal–insulator transition concomitant with a structural phase transition to a distorted rutile phase, akin to NbO2. Our study expands the understanding of tantalum oxides and paves the way for their integration into next-generation electronic and photonic devices.

Perovskite oxides are a versatile class of materials with tunable electronic structures, making them attractive for catalytic applications, including the oxygen evolution reaction (OER). The surface reactivity of these oxides is closely tied to the electronic structure of transition metal cations, particularly their 3d orbital occupation, which can be modulated by interfacial engineering. In this work, we investigate how subsurface engineering influences the interaction of ultrathin LaCoO₃ films with water vapor. Using (near) ambient pressure core-level spectroscopy, we observe distinct differences in hydroxyl affinity and Co valence response depending on the electronic structure imposed by the underlying layer. Ultrathin LaCoO₃ films with a higher initial Co oxidation state show stronger hydroxyl affinity, while those with a lower Co valence show more significant electronic changes upon water exposure. Our findings demonstrate a form of "remote control" in surface chemistry, where subsurface electronic engineering dictates hydroxyl affinity and electronic response at the surface. This concept offers a new degree of freedom to optimize oxide-adsorbate interactions for (electro)catalysis.

We investigated the valence dynamics of thin films during the electrocatalytic oxygen evolution reaction using operando high-energy-resolution fluorescence-detected (HERFD) X-ray absorption spectroscopy (XAS) in a 0.1 M KOH solution under an applied potential. We show that it is possible to measure the 10 unit cell (∼4 nm) bimetallic perovskite oxide La₂CoMnO₆ thin film grown on Pt-covered SiN_x membranes. Operando Co and Mn K-edge HERFD XAS spectra on the active surface species were recorded from the backside of the SiN_x membrane. The HERFD XAS Co K-edge spectra reveal a (partial) increase in the oxidation state from Co²⁺ in ex situ conditions to mainly Co³⁺ in the solid-liquid open-circuit voltage (OCV). Voltage-dependent Co K-edge HERFD XAS white line intensities show that Co is oxidized toward the higher oxidation state, while Mn remains Mn⁴⁺. Interestingly, the Co and Mn main edge shifts with the average valence change of 0.20; the valence of Mn is constant, implying that the valence of Co increases by 0.40. The OCV spectra were reversible when switching off the potentials. These observations clearly identify element-specific valence dynamics in the intrinsic oxygen evolution reaction.

Photoexcited states of tungsten trioxide (WO₃) were observed using femtosecond high-energy-resolution fluorescence detection X-ray Absorption Spectra (HERFD-XAS) and resonant X-ray emission spectra (RXES). In the initial state of the photoexcitation (∼100 fs), the W L₃ edge XAS shifts to lower energy and the energy levels of the t_2g and e_g peaks are modulated due to the photoexcited electrons in the conduction band. The electronic state of the photoexcited W atoms is modified by 500 fs. The crystal field splitting between the t_2g and e_g peaks shrinks by 500 fs, which indicates local structural changes around the W atoms due to the formation of polarons. HERFD-XAS and RXES provide more details about the early state of the photoexcited states of WO₃. This work demonstrates that the detailed dynamics of 5d elements in the femtosecond range can be addressed with HERFD-XAS/RXES.

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).

Conducting real‐time, element‐specific studies of photo‐excited systems is a long‐standing challenge. The development of X‐ray free‐electron lasers (XFELs) has paved the way for the emergence of a promising technique: femtosecond X‐ray absorption spectroscopy (fs‐XAS). This powerful technique reveals electronic and geometric characteristics, providing unprecedented insight into their dynamic interactions under nonequilibrium conditions. Herein, the fs‐XAS technique is employed at PAL‐XFEL to unravel light‐driven ultrafast electronic and structural changes in epitaxial lanthanum iron oxide (LaFeO₃) thin films. Density functional theory (DFT) and multiplet calculations are utilized to expound on the experimental results. The analyses reveal that photoexcitation initially induces high‐ and intermediate‐spin Fe²⁺ states through ligand‐to‐metal charge transfer (LMCT), followed by polaron formation. It is demonstrated that the reduced overlap between the oxygen 2p and iron 3d orbitals accounts for all experimental observations, including 1) the XAS shifts to lower energies, 2) the decrease in the crystal field splitting, and 3) the relatively larger shifts observed in the oxygen 1s XAS.