Inelastic X-ray Spectroscopy Research Articles

Oxygen-Redox Activity in Non-Lithium-Excess Tungsten-Doped LiNiO2 Cathode

The desire to increase the energy density of stoichiometric layered LiTMO2 (TM = 3d transition metal) cathode materials has promoted investigation into their properties at high states of charge. Although there is increasing evidence for pronounced oxygen participation in the charge compensation mechanism, questions remain whether this is true O-redox, as observed in Li-excess cathodes. Through a high-resolution O K-edge resonant inelastic x-ray spectroscopy (RIXS) study of the Mn-free Ni-rich layered oxide LiNi0.98W0.02O2, we demonstrate that the same oxidized oxygen environment exists in both Li-excess and non-Li-excess systems. The observation of identical RIXS loss features in both classes of compounds is remarkable given the differences in their crystallographic structure and delithiation pathways. This lack of a specific structural motif reveals the importance of electron correlation in the charge compensation mechanism for these systems and indicates how a better description of charge compensation in layered oxides is required to understand anionic redox for energy storage.Received 21 October 2022Revised 7 February 2023Accepted 21 February 2023DOI:https://doi.org/10.1103/PRXEnergy.2.013005Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectrochemical propertiesMixed ionic electronic conductionPhysical SystemsEnergy applicationsEnergy materialsTechniquesX-ray absorption spectroscopyX-ray diffractionX-ray emission spectroscopyEnergy Science & TechnologyCondensed Matter, Materials & Applied Physics

Tailoring Redox Reactions for Stable Cycling of High-Capacity Li-Ion Cathodes

Development of new cathode materials with even better performance and lower cost is needed to meet the ever-increasing global demand on lithium-ion batteries. As such, extensive efforts have been devoted to developing next-generation cathodes for high-energy Li-ion batteries, e.g., Li-rich transition metal oxides. As a counterpart to the conventional cation-ordered layered oxides, cation-disordered rocksalts (DRXs) have emerged as a new class of high-capacity cathode materials. One unique advantage of the DRX chemistry is the structural flexibility that substantially lessens the elemental constraints and enables the incorporation of a wide variety of TMs and fluorine anion at a decent amount in the crystal lattice. As a result, the dominant TMs have been extended from redox-active Ni and Co in layered cathodes to cost-effective and earth-abundant Mn and Fe as well as redox-inactive TM (e.g., Nb, Ti, Zr, Mo) to facilitate the DRX phase formation. This exceptional chemical diversity largely opens up the opportunity to tune the material compositions for optimal performance through the control of redox reactions.In this talk, I will present our recent work on several high-capacity cathodes based on Mn and Ni redox centers. We specifically focused on understanding the redox chemistry, including cationic and anionic redox, using combined electrochemical, soft X-ray absorption (XAS), and resonant inelastic X-ray spectroscopy (RIXS) techniques. This presentation summarizes the different TM and O redox behaviors and their roles in the electrochemical processes of these high-capacity cathodes. I will then show some examples on how this understanding leads to improved cathodes with enhanced performance by tailoring the redox reactions.

Charge Order and Superconductivity as Competing Brothers in Cuprate High-Tc Superconductors

Studies on the interplay between the charge order and the $d$-wave superconductivity in the copper-oxide high $T_{\rm c}$ superconductors are reviewed with a special emphasis on the exploration based on the unconventional concept of the electron fractionalization and its consequences supported by solutions of high-accuracy quantum many-body solvers. Severe competitions between the superconducting states and the charge inhomogeneity including the charge/spin striped states revealed by the quantum many-body solvers are first addressed for the Hubbard models and then for the {\it ab initio} Hamiltonians of the cuprates derived without adjustable parameters to represent the low-energy physics of the cuprates. The charge inhomogeneity and superconductivity are born out of the same mother, namely, the carrier attraction arising from the strong Coulomb repulsion near the Mott insulator (Mottness) and accompanied electron fractionalization. The same mother makes the severe competition of the two brothers inevitable. The electron fractionalization has a remarkable consequences on the mechanism of the superconductivity. Recent explorations motivated by the concept of the fractionalization and their consequences on experimental observations in energy-momentum resolved spectroscopic measurements including the angle resolved photoemission spectroscopy (ARPES) and the resonant inelastic X-ray spectroscopy (RIXS) are overviewed, with future vision for the integrated spectroscopy to challenge the long-standing difficulties in the cuprates as well as in other strongly correlated matter in general.

(Invited) Anionic Redox Chemistry in Layered Oxide Cathodes

Li-ion battery remains as the critical energy storage device and starts to invade the large electrical vehicle (EV) market. Extensive efforts have been devoted to developing advanced layered oxide cathodes, including both stoichiometric and Li-rich layered oxides, for Li-ion batteries of even better performance. For all these cathodes, there is a clear need for the high-voltage cycling to obtain an even higher practical capacity. However, the redox reactions become more complex when charged at high voltages. Beyond the conventional transition metal (TM) redox, anionic oxygen becomes active and participates in the electrochemical reaction in these cathodes. The anionic oxygen activity involves both the reversible oxygen redox and irreversible oxygen loss accompanied by other byproducts resulting from the undesirable side reactions at high voltages.In the past, my group has been focusing on understanding the redox chemistry, including both cationic and anionic redox reactions, in both stoichiometric and Li-rich layered oxide cathodes, especially towards the high-voltage cycling. We specifically investigated the electronic states of TM and O using combined electrochemical, soft X-ray absorption (XAS), resonant inelastic X-ray spectroscopy (RIXS) and in situ differential electrochemical mass spectrometry (DEMS) in the high voltage region to elucidate the explicit redox chemistry in various layered oxide cathodes. This presentation will highlight a summary on the different TM and O redox behaviors in these technologically relevant cathode materials for a close comparison. Of particular interest is the fundamental understanding of how the anionic oxygen participates in the electrochemistry of layered oxides with the presence of different Li and TM stoichiometry.

Observation of a chiral wave function in the twofold-degenerate quadruple Weyl system BaPtGe

Topological states in quantum materials are defined by non-trivial topological invariants, such as the Chern number, which are properties of their bulk wave functions. A remarkable consequence of topological wave functions is the emergence of edge modes, a phenomenon known as bulk-edge correspondence, that gives rise to quantized or chiral physical properties. While edge modes are widely presented as signatures of non-trivial topology, how bulk wave functions can manifest explicitly topological properties remains unresolved. Here, using high-resolution inelastic x-ray spectroscopy (IXS) combined with first principles calculations, we report experimental signatures of chiral wave functions in the bulk phonon spectrum of BaPtGe, which we show to host a previously undiscovered twofold degenerate quadruple Weyl node. The chirality of the degenerate phononic wave function yields a non-trivial phonon dynamical structure factor, S(Q,$\omega$), along high-symmetry directions, that is in excellent agreement with numerical and model calculations. Our results establish IXS as a powerful tool to uncover topological wave functions, providing a key missing ingredient in the study of topological quantum matter.

Resonant inelastic x-ray spectroscopy on UO2 as a test case for actinide materials

Resonant inelastic x-ray spectroscopy at the uranium N4 absorption edge at 778 eV has been used to reveal the excitations in UO2 up to 1 eV. The earlier (1989) studies by neutron inelastic scattering of the crystal-field states within the 3H4 multiplet are confirmed. In addition, the first excited state of the 3F2 multiplet at ∼520 meV has been established, and there is a weak signal corresponding to the next excited state at ∼920 meV. This represents a successful application of soft x-ray spectroscopy to an actinide sample, and resolves an open question in UO2 that has been discussed for 50 years. The technique is described and important caveats are drawn about possible future applications.

Anisotropic optical phonons in MOCVD grown Si-doped GaN/Sapphire epilayers

Micro-Raman spectroscopy is employed to study the anisotropic optical phonons of Si-doped GaN/Sapphire epifilms grown by metal organic chemical vapor deposition method. In an undoped 3.6 μm thick sample – our polarized Raman measurements in the backscattering geometry revealed major first order modes of GaN and sapphire. Careful analyses of the second-order Raman spectra using critical-point-phonons from a rigid-ion-model fitted inelastic X-ray spectroscopy data with appropriate selection rules helped us attain expedient data for the lattice dynamics of GaN. In Si-doped films, a modified phonon confinement model is used for simulating Raman line shapes of E2high phonons to monitor crystalline quality. While the optical phonons in lightly doped samples are coupled to electron plasma– at higher carrier concentration the over-damped A1(LO) mode vanished in the background. For each sample we assessed the transport parameters by simulating Raman profiles of A1(LO) line shape with contributions from plasmon-LO-phonon and Lorentzian shaped Eg sapphire mode. A realistic Green’s function theory is adopted to study the vibrational modes of Si donors and Mg acceptors in GaN by including force constant changes estimated from lattice relaxations using first-principles bond-orbital model. Theoretical results of impurity-activated modes compared favorably well with the existing Raman scattering data.

Bethe–Salpeter equation for absorption and scattering spectroscopy: implementation in the exciting code

The Bethe–Salpeter equation for the electron–hole correlation function is the state-of-the-art formalism for optical and core spectroscopy in condensed matter. Solutions of this equation yield the full dielectric response, including both the absorption and the inelastic scattering spectra. Here, we present an efficient implementation within the all-electron full-potential code exciting, which employs the linearized augmented plane-wave (L)APW+LO basis set. Being an all-electron code, exciting allows the calculation of optical and core excitations on the same footing. The implementation fully includes the effects of finite momentum transfer which may occur in inelastic x-ray spectroscopy and electron energy-loss spectroscopy. Our implementation does not require the application of the Tamm–Dancoff approximation that is commonly employed in the determination of absorption spectra in condensed matter. The interface with parallel linear-algebra libraries enables the calculation for complex systems. The capability of our implementation to compute, analyze, and interpret the results of different spectroscopic techniques is demonstrated by selected examples of prototypical inorganic and organic semiconductors and insulators.

Redox Chemistry in Conventional Layered Lithium Metal Oxide Cathodes

Li-ion battery technology is one of the best performing energy storage devices and holds the best promise to power the upcoming electrical vehicles (EV). Extensive efforts have been devoted to further advancing conventional layered oxide cathodes, such as LiNixMnyCozO2 and LiNixCoyAlzO2 (0 < x, y, z <1), for Li-ion batteries of even better performance and lower cost ($/Wh). Strategies that are largely pursued include increasing Ni content to achieve a higher practical capacity at a given charge cutoff voltage and/or increasing charge cutoff voltage for a given composition. Alongside with these research efforts, questions regarding the stability of electrode, especially at highly delithiated states, electrolyte and electrode/electrolyte interface arise. In this work, of particular interest is the redox chemistry upon electrochemical (de)lithiation of LiNiO2, the parent compound of Ni-rich layered oxides, with a key focus on highly delithiated states in the high voltage region. Electrochemistry of LiNiO2 seems straightforward, in principle, Ni3+/Ni4+ redox can perfectly accommodate 1 Li+ transport during (de)lithiation process. However, the precise state at full oxidation of Ni3+ to Ni4+ and/or full removal of 1 Li+ are difficult to achieve because of the possible side reactions between highly charged electrode and electrolyte, for example, surface Ni reduction and gas evolution upon charge often occur at high voltages. Here, the electronic states of Ni and O are probed at different states of charge through combined soft X-ray absorption (XAS), resonant inelastic X-ray spectroscopy (RIXS) and in situ differential electrochemical mass spectrometry (DEMS) in the high voltage region to elucidate the explicit redox chemistry in LiNiO2. This work is anticipated to provide additional insights into the redox chemistry of conventional Ni-rich layered oxides, especially its utilization in the high voltage region.

Resonant inelastic X-ray scattering of tantalum double perovskite structures

In this paper, we investigated the electronic structures and defect states of SrLaMgTaO6 (SLMTO) double perovskite structures by using resonant inelastic x-ray scattering. Recently, Eu3+ doped SLMTO red phosphors have been vigorously investigated due to their higher red emission efficiency compared to commercial white light emitting diodes (W-LED). However, a comprehensive understanding on the electronic structures and defect states of host SLMTO compounds, which are specifically related to the W-LED and photoluminescence (PL), is far from complete. Here, we found that the PL spectra of SLMTO powder compounds sintered at a higher temperature, 1400 °C, were weaker in the blue emission regions (at around 400 nm) and became enhanced in near infrared (NIR) regions compared to those sintered at 1200 °C. To elucidate the difference of the PL spectra, we performed resonant inelastic x-ray spectroscopy (RIXS) at Ta L-edge. Our RIXS result implies that the microscopic origin of different PL spectra is not relevant to the Ta-related defects and oxygen vacancies.

Anion Redox Chemistry in the Cobalt Free 3d Transition Metal Oxide Intercalation Electrode Li[Li0.2Ni0.2Mn0.6]O2.

Conventional intercalation cathodes for lithium batteries store charge in redox reactions associated with the transition metal cations, e.g., Mn(3+/4+) in LiMn2O4, and this limits the energy storage of Li-ion batteries. Compounds such as Li[Li0.2Ni0.2Mn0.6]O2 exhibit a capacity to store charge in excess of the transition metal redox reactions. The additional capacity occurs at and above 4.5 V versus Li(+)/Li. The capacity at 4.5 V is dominated by oxidation of the O(2-) anions accounting for ∼0.43 e(-)/formula unit, with an additional 0.06 e(-)/formula unit being associated with O loss from the lattice. In contrast, the capacity above 4.5 V is mainly O loss, ∼0.08 e(-)/formula. The O redox reaction involves the formation of localized hole states on O during charge, which are located on O coordinated by (Mn(4+)/Li(+)). The results have been obtained by combining operando electrochemical mass spec on (18)O labeled Li[Li0.2Ni0.2Mn0.6]O2 with XANES, soft X-ray spectroscopy, resonant inelastic X-ray spectroscopy, and Raman spectroscopy. Finally the general features of O redox are described with discussion about the role of comparatively ionic (less covalent) 3d metal-oxygen interaction on anion redox in lithium rich cathode materials.

Effect of Pressure on Valence and Structural Properties of YbFe2Ge2 Heavy Fermion Compound--A Combined Inelastic X-ray Spectroscopy, X-ray Diffraction, and Theoretical Investigation.

The crystal structure and the Yb valence of the YbFe2Ge2 heavy fermion compound was measured at room temperature and under high pressures using high-pressure powder X-ray diffraction and X-ray absorption spectroscopy via both partial fluorescence yield and resonant inelastic X-ray emission techniques. The measurements are complemented by first-principles density functional theoretical calculations using the self-interaction corrected local spin density approximation investigating in particular the magnetic structure and the Yb valence. While the ThCr2Si2-type tetragonal (I4/mmm) structure is stable up to 53 GPa, the X-ray emission results show an increase of the Yb valence from v = 2.72(2) at ambient pressure to v = 2.93(3) at ∼9 GPa, where at low temperature a pressure-induced quantum critical state was reported.

Electron-Phonon Coupling in the Bulk of Anatase TiO2 Measured by Resonant Inelastic X-Ray Spectroscopy.

We investigate the polaronic ground state of anatase TiO2 by bulk-sensitive resonant inelastic x-ray spectroscopy (RIXS) at the Ti L3 edge. We find that the formation of the polaron cloud involves a single 95 meV phonon along the c axis, in addition to the 108 meV ab-plane mode previously identified by photoemission. The coupling strength to both modes is the same within error bars, and it is unaffected by the carrier density. These data establish RIXS as a directional bulk-sensitive probe of electron-phonon coupling in solids.

Resonant inelastic X-ray spectroscopy of atoms and simple molecules: Satellite features and dependence on energy detuning and photon polarization

We summarize recent results dealing with high resolution (resonant) X-ray spectroscopy of atomic and molecular targets in the tender X-ray energy region. We comment on advantages, new possibilities and problems related to RIXS spectroscopy with respect to the standard photoabsorption technique, where scanning the probe energy is the only option. In particular, three research areas are covered: X-ray emission mediated by energy dependent photoabsorption to multi-electron excited states, the Cl K core-hole clock studies exemplified by systematic study of chloro(fluoro)-hydrocarbon targets and the polarization dependent X-ray emission studies. Due to its spectral selectivity and simultaneous detection capability, high resolution wavelength dispersive X-ray spectroscopy has the capability to resolve structural and dynamical properties of matter within new instrumentation frontiers.

Electronic Structure of Monoclinic BiVO4

A comprehensive approach to understanding the electronic structure of monoclinic scheelite bismuth vanadate (ms-BiVO4), including both valence band (VB) and conduction band (CB) orbital character, is presented. Density functional theory (DFT) calculations are directly compared to experimental data obtained via X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy, resonant inelastic X-ray spectroscopy (RIXS), and X-ray photoelectron spectroscopy to provide a complete portrait of the total and partial density of states (DOS) near the bandgap. DFT calculations are presented to confirm the VB maximum and CB minimum are comprised primarily of O 2p and V 3d orbitals, respectively. Predicted triplet d-manifold splitting of V 3d CB states, arising from lone pair-induced lattice distortions, is quantified by V L- and O K-edge XAS. Furthermore, the partial contributions to the total DOS within both the CB and VB, determined by RIXS, are found to be in excellent agreement with DFT calculations. Energy levels are placed relative to the vacuum level by photoemission spectroscopy, which provides a measure of the work function and electron affinity of the investigated BiVO4 thin film. The implications of the fundamental electronic structure of ms-BiVO4 on its photocatalytic behavior, as well as considerations for improvements by substitutional incorporation of additional elements, are discussed.

Multi-spin-state dynamics during insulator-metal crossover inLaCoO3

We have investigated the dynamics of spin-state crossover (SSC) for perovskite ${\mathrm{LaCoO}}_{3}$ through optical phonons by means of infrared and inelastic x-ray spectroscopy. Upon thermally induced SSC, anomalously dispersionless Co-O bond stretching phonons coupled to the thermally excited spin state have been identified. The enhanced spin-state fluctuation irrespective of the presence of a clear charge gap suggests the emergence of complex spin-state disproportionation involving low-, intermediate-, and high-spin states due to the strong correlation among thermally activated spin states.

Hybrid diamond-silicon angular-dispersive x-ray monochromator with 025-meV energy bandwidth and high spectral efficiency

We report on the design, implementation, and performance of an x-ray monochromator with ultra-high energy resolution (ΔE/E ≃ 2.7 × 10(-8)) and high spectral efficiency using x rays with photon energies E ≃ 9.13 keV. The operating principle of the monochromator is based on the phenomenon of angular dispersion in Bragg back-diffraction. The optical scheme of the monochromator is a modification of a scheme reported earlier [Shvyd'ko et al., Phys. Rev. A 84, 053823 (2011)], where a collimator/wavelength selector Si crystal was replaced with a 100-μm-thick type IIa diamond crystal. This modification provides a very-small-energy bandwidth ΔE ≃ 0.25 meV, a 3-fold increase in the aperture of the accepted beam, a reduction in the cumulative angular dispersion rate of x rays emanating from the monochromator for better focusing on a sample, a sufficient angular acceptance matching the angular divergence of an undulator source (≈ 10 μrad), and an improved throughput due to low x-ray absorption in the thin diamond crystal. The measured spectral efficiency of the monochromator was ≈ 65% with an aperture of 0.3 × 1 mm(2). The performance parameters of the monochromator are suitable for inelastic x-ray spectroscopy with an absolute energy resolution ΔE < 1 meV.

New Energy Sources: in-situ Characterisation of Fuel Cell and Supercapacitor Components. Complementary Studies using Transmission, Fluorescence and Photoelectron Microscopy and Imaging

Fuel cells and supercapacitors are electrochemical devices providing efficient and pollution-free production and transformation of electricity. Notwithstanding their environmental appeal, a host of materials-science problems – chiefly related to the limited durability of crucial functional components – are hindering their widespread application. The present knowledge of the relevant materials-science notion is mostly at the macroscopic and empirical trial-and-error level and the answers to many questions require much deeper scientific understanding of the origin of degradation processes. In this regard, the development and the implementation of appropriate methods for in-situ characterization of cell components at the functionally relevant length scales is highly required. Soft X-ray spectroscopies, such as X-ray absorption spectroscopy, X-ray emission (fluorescence) spectroscopy, resonant inelastic X-ray spectroscopy and X-ray photoelectron spectroscopy have been extensively employed for ex-situ characterization of materials used in electrochemical systems. Furthermore, adding spatial resolution capabilities by implementing proper optical solutions has opened unique opportunities for monitoring material changes and mass transport events occurring at submicron length scales. The input from these methods is providing correlative information about the status of the electrode surface and of the electrode/electrolyte interface and also of the processes occurring under operation conditions.

Fast electron dynamics in vanadates measured by resonant inelastic x-ray scattering

We present experimental data for the decay dynamics of a resonantly-produced 2p1/2 vanadium core-hole in YVO4. The states produced by the fast Coster–Kronig decay are studied by resonant inelastic x-ray spectroscopy. They form groups of participant normal emission and spectator emission peaks. The spectator emission gives rise to sharp peaks whose shape and energy correspond to a screened resonant 2p3/2↔3d transition. This behavior is compared with the emission features observed in the less localized system V2O5. The decay lifetime of the 2p1/2 hole in YVO4 is measured by the broadening of the sharp peaks resulting in a value of the order of 1fs.

Pressure-Induced Valence and Structural Changes in YbMn2Ge2—Inelastic X-ray Spectroscopy and Theoretical Investigations

The pressure-induced valence change of Yb in YbMn(2)Ge(2) has been studied by high pressure inelastic X-ray emission and absorption spectroscopy in the partial fluorescence yield mode up to 30 GPa. The crystal structure of YbMn(2)Ge(2) has been investigated by high pressure powder X-ray diffraction experiments up to 40 GPa. The experimental investigations have been complemented by first principles density functional theoretical calculations using the generalized gradient approximation with an evolutionary algorithm for structural determination. The Yb valence and magnetic structures have been calculated using the self-interaction corrected local spin density approximation. The X-ray emission results indicate a sharp increase of Yb valence from v = 2.42(2) to v = 2.75(3) around 1.35 GPa, and Yb reaches a near trivalent state (v = 2.95(3)) around 30 GPa. Further, a new monoclinic P1 type high pressure phase is found above 35 GPa; this structure is characterized by the Mn layer of the ambient (I4/mmm) structure transforming into a double layer. The theoretical calculations yield an effective valence of v = 2.48 at ambient pressure in agreement with experiment, although the pure trivalent state is attained theoretically at significantly higher pressures (above 40 GPa).