Resonant Inelastic X-ray Scattering Spectroscopy Research Articles

A New High-Valent Fe Redox Couple in Intercalation Electrodes

The overwhelming majority of layered oxide cathode materials are based on the redox activity of expensive and scarce transition metals like Co and Ni. While the Fe-redox based material LiFePO4 has become increasingly attractive due to its low cost, its low gravimetric energy density and relatively low operating voltage limit its use cases. A high-voltage Fe-based layered oxide cathode would provide high energy density utilizing the redox-activity of the most abundant transition metal in Earth’s crust, but such a material that provides stable and reversible electrochemical performance remains elusive. Indeed, high-voltage Fe-redox in oxides is plagued by cation disorder and oxygen dimerization, which leads to capacity fading and large voltage hysteresis (> 1 V).[1]In contrast, the layered oxide cathode Li4FeSbO6 (equivalently, Li[Li1/3Fe1/3Sb1/3]O2, abbreviated LFSO) exhibits <0.2 V of hysteresis, and a discharge plateau at 4.2 V versus Li/Li+ with two-electron capacity.[2] Both the exceptional electrochemical reversibility of this material and its phase separating nature upon (de)intercalation raise questions about the origin of these properties that are unusual for layered Fe oxide cathodes. In this work, we will discuss our efforts to probe the electronic structure of LFSO with Fe L–edge x–ray absorption (XAS) and 57Fe Mössbauer spectroscopy, as well as O K–edge resonant inelastic x–ray scattering spectroscopy (RIXS). The combination of these element-specific and valence-state sensitive techniques, in conjunction with charge-transfer multiplet and density functional theory calculations, allows us to reveal the electronic structure changes at Fe and O as a function of state of charge for the first time. Correlating these electronic structure changes with synchrotron diffraction measurements, we reveal a unique redox compensation mechanism that readily rationalizes the exceptional electrochemical reversibility, unlocking a new direction for high-voltage, Fe-based redox systems.

Time-Resolved X-ray Emission Spectroscopy and Resonant Inelastic X-ray Scattering Spectroscopy of Laser Irradiated Carbon.

The existence of liquid carbon as an intermediate phase preceding the formation of novel carbon materials has been a point of contention for several decades. Experimental observation of such a liquid state requires nonthermal melting of solid carbon materials at various laser fluences and pulse properties. Reflectivity experiments performed in the mid-1980s reached opposing conclusions regarding the metallic or insulating properties of the purported liquid state. Time-resolved X-ray absorption studies showed shortening of C-C bonds and increasing diffraction densities, thought to evidence a liquid or glassy carbon state, respectively. Nevertheless, none of these experiments provided information on the electronic structure of the proposed liquid state. Herein, we report the results of time-resolved resonant inelastic X-ray scattering (RIXS) and time-resolved X-ray emission spectroscopy (XES) studies on amorphous carbon (a-C) and ultrananocrystalline diamond (UNCD) as a function of delay time between the irradiating pulse and X-ray probe. For both a-C and UNCD, we attribute decreases in RIXS or XES signals to transition blocking, relaxation, and finally, ablation. Increased signal at 20 ps following the irradiation of the UNCD is attributed to the probable formation of nanoscale structures in the ablation plume. Differences in the amount of signal observed between a-C and UNCD are explained by the difference in sample thickness and, specifically, incomplete melting of the UNCD film. Comparisons to spectral simulations based on MD trajectories at extreme conditions indicate that the carbon state in our experiments is crystalline. Normal mode analysis confirmed that symmetrical bending or stretching of the C-C bonds in the diamond lattice results in XES spectra with small intensity differences. Overall, we observed no evidence of melting to a liquid state, as determined by the lack of changes in the spectral properties for up to 100 ps delays following the melting pulses.

Measurement of the Ligand Field Spectra of Ferrous and Ferric Iron Chlorides Using 2p3d RIXS.

Ligand field spectra provide direct information about the electronic structure of transition metal complexes. However, these spectra are difficult to measure by conventional optical techniques due to small cross sections for d-to-d transitions and instrumental limitations below 4000 cm-1. 2p3d resonant inelastic X-ray scattering (RIXS) is a second order process that utilizes dipole allowed 2p to 3d transitions to access d-d excited states. The measurement of ligand field excitation spectra by RIXS is demonstrated for a series of tetrahedral and octahedral Fe(II) and Fe(III) chlorides, which are denoted Fe(III)-Td, Fe(II)-Td, Fe(III)-Oh, and Fe(II)-Oh. The strong 2p spin-orbit coupling allows the measurement of spin forbidden transitions in RIXS spectroscopy. The Fe(III) spectra are dominated by transitions from the sextet ground state to quartet excited states, and the Fe(II) spectra contain transitions to triplet states in addition to the spin allowed 5Γ → 5Γ transition. Each experimental spectrum is simulated using a ligand field multiplet model to extract the ligand field splitting parameter 10Dq and the Racah parameters B and C. The 10Dq values for Fe(III)-Td, Fe(II)-Td, and Fe(III)-Oh are found to be -0.7, -0.32, and 1.47 eV, respectively. In the case of Fe(II)-Oh, a single 10Dq parameter cannot be assigned because Fe(II)-Oh is a coordination polymer exhibiting axially compressed Fe(II)Cl 6 units. The 5T → 5E transition is split by the axial compression resulting in features at 0.51 and 0.88 eV. The present study forms the foundation for future applications of 2p3d RIXS to molecular iron sites in more complex systems, including iron-based catalysts and enzymes.

Resonant Inelastic X-ray Scattering Spectroscopy at MERLIN Beamline at the Advanced Light Source

Strong correlations between charge, spin, orbital, and lattice degrees of freedom in complex materials can often lead to the emergence of exotic properties, such as high temperature superconductivity (HTSC), colossal magnetoresistance (CMR), multiferroicity, insulator-to-metal transition (IMT), electron fractionalization, and many more [1–12]. These properties not only show great promise for the technological breakthroughs that can transform our way of life, but also provide ideal test grounds for exploring the fundamental physics of electronic correlations on various energy, length, and time scales.

X-Ray absorption in homogeneous catalysis research: the iron-catalyzed Michael addition reaction by XAS, RIXS and multi-dimensional spectroscopy

A survey over X-ray absorption methods in homogeneous catalysis research is given with the example of the iron-catalyzed Michael addition reaction. A thorough investigation of the catalytic cycle was possible by combination of conventional X-ray absorption spectroscopy (XAS), resonant inelastic X-ray scattering (RIXS) and multi-dimensional spectroscopy. The catalytically active compound formed in the first step of the Michael reaction of methyl vinyl ketone with 2-oxocyclopentanecarboxylate (1) could be elucidated in situ by RIXS spectroscopy, and the reduced catalytic activity of FeCl(3) x 6 H(2)O (2) compared to Fe(ClO(4))(3) x 9 H(2)O (3) could be further explained by the formation of a [Fe(III)Cl(4)(-)](3)[Fe(III)(1-H)(2)(H(2)O)(2)(+)][H(+)](2) complex. Chloride was identified as catalyst poison with a combined XAS-UV/vis study, which revealed that Cl(-) binds quantitatively to the available iron centers that are deactivated by formation of [FeCl(4)(-)]. Operando studies in the course of the reaction of methyl vinyl ketone with 1 by combined XAS-Raman spectroscopy allowed the exclusion of changes in the oxidation state and the octahedral geometry at the iron site; a reaction order of two with respect to methyl vinyl ketone and a rate constant of k = 1.413 min(-2) were determined by analysis of the C=C and C=O vibration band. Finally, a dedicated experimental set-up for three-dimensional spectroscopic studies (XAS, UV/vis and Raman) of homogeneous catalytic reactions under laboratory conditions, which emerged from the discussed investigations, is presented.

Where Water Is Oxidized to Dioxygen: Structure of the Photosynthetic Mn4Ca Cluster from X-ray Spectroscopy

Light-driven oxidation of water to dioxygen in plants, algae, and cyanobacteria is catalyzed within photosystem II (PS II) by a Mn 4Ca cluster. Although the cluster has been studied by many different methods, its structure and mechanism have remained elusive. X-ray absorption and emission spectroscopy and extended X-ray absorption fine structure studies have been particularly useful in probing the electronic and geometric structures and the mechanism of the water oxidation reaction. Recent progress, reviewed here, includes polarized X-ray absorption spectroscopy measurements of PS II single crystals. Analysis of those results has constrained the Mn 4Ca cluster geometry to a set of three similar high-resolution structures. The structure of the cluster from the present study is unlike either the 3.0- or 3.5-A-resolution X-ray structures or other previously proposed models. The differences between the models derived from X-ray spectroscopy and crystallography are predominantly because of damage to the Mn 4Ca cluster by X-rays under conditions used for the structure determination by X-ray crystallography. X-ray spectroscopy studies are also used for studying the changes in the structure of the Mn 4Ca catalytic center as it cycles through the five intermediate states known as the S i states ( i = 0-4). The electronic structure of the Mn 4Ca cluster has been studied more recently using resonant inelastic X-ray scattering spectroscopy (RIXS), in addition to the earlier X-ray absorption and emission spectroscopy methods. These studies are revealing that the assignment of formal oxidation states is overly simplistic. A more accurate description should consider the charge density on the Mn atoms, which includes the covalency of the bonds and delocalization of the charge over the cluster. The geometric and electronic structures of the Mn 4Ca cluster in the S states derived from X-ray spectroscopy are leading to a detailed understanding of the mechanism of O-O bond formation during the photosynthetic water-splitting process.

Electron correlation effects in resonant inelastic X-ray scattering of NaV2O5.

Element- and site-specific resonant inelastic x-ray scattering spectroscopy (RIXS) is employed to investigate electron correlation effects in NaV2O5. In contrast to single photon techniques, RIXS at the vanadium L3 edge is able to probe d-d* transitions between V d-bands. A sharp energy loss feature is observed at -1.56 eV, which is well reproduced by a model calculation including correlation effects. The calculation identifies the loss feature as excitation between the lower and upper Hubbard bands and permits an accurate determination of the Hubbard interaction term U = 3.0 +/- 0.2 eV.

Electronic excitations investigated by inelastic x-ray scatteringspectroscopy

It is shown how non-resonant and resonant inelastic x-ray scatteringspectroscopy (IXSS) offer valuable information about electronic excitations insolids. After a presentation of the basic relations, applications of IXSS on alarge variety of systems and electronic excitations are discussed, where inmost cases synchrotron radiation is utilized