The duality of electron localization and covalency in lanthanide and actinide metallocenes.

Previous magnetic, spectroscopic, and theoretical studies of cerocene, Ce(C8H8)2, have provided evidence for non-negligible 4f-electron density on Ce and implied that charge transfer from the ligands occurs as a result of covalent bonding. Strong correlations of the localized 4f-electrons to the delocalized ligand π-system result in emergence of Kondo-like behavior and other quantum chemical phenomena that are rarely observed in molecular systems. In this study, Ce(C8H8)2 is analyzed experimentally using carbon K-edge and cerium M5,4-edge X-ray absorption spectroscopies (XAS), and computationally using configuration interaction (CI) calculations and density functional theory (DFT) as well as time-dependent DFT (TDDFT). Both spectroscopic approaches provide strong evidence for ligand → metal electron transfer as a result of Ce 4f and 5d mixing with the occupied C 2p orbitals of the C8H8 2- ligands. Specifically, the Ce M5,4-edge XAS and CI calculations show that the contribution of the 4f1, or Ce3+, configuration to the ground state of Ce(C8H8)2 is similar to strongly correlated materials such as CeRh3 and significantly larger than observed for other formally Ce4+ compounds including CeO2 and CeCl6 2-. Pre-edge features in the experimental and TDDFT-simulated C K-edge XAS provide unequivocal evidence for C 2p and Ce 4f covalent orbital mixing in the δ-antibonding orbitals of e2u symmetry, which are the unoccupied counterparts to the occupied, ligand-based δ-bonding e2u orbitals. The C K-edge peak intensities, which can be compared directly to the C 2p and Ce 4f orbital mixing coefficients determined by DFT, show that covalency in Ce(C8H8)2 is comparable in magnitude to values reported previously for U(C8H8)2. An intuitive model is presented to show how similar covalent contributions to the ground state can have different impacts on the overall stability of f-element metallocenes.

The electronic structure of f-element Prussian blue analogs determined by soft X-ray absorption spectroscopy.

In molecular solids derived from Prussian blue, intermetallic charge transfer is fostered through a cyano bridge two metal ions. In this study, isostructural trivalent lanthanide and tetravalent actinide Prussian blue analogs' valence orbitals are probed by soft X-ray absorption measurements.

Three-dimensional localization of nanoscale battery reactions using soft X-ray tomography.

Battery function is determined by the efficiency and reversibility of the electrochemical phase transformations at solid electrodes. The microscopic tools available to study the chemical states of matter with the required spatial resolution and chemical specificity are intrinsically limited when studying complex architectures by their reliance on two-dimensional projections of thick material. Here, we report the development of soft X-ray ptychographic tomography, which resolves chemical states in three dimensions at 11 nm spatial resolution. We study an ensemble of nano-plates of lithium iron phosphate extracted from a battery electrode at 50% state of charge. Using a set of nanoscale tomograms, we quantify the electrochemical state and resolve phase boundaries throughout the volume of individual nanoparticles. These observations reveal multiple reaction points, intra-particle heterogeneity, and size effects that highlight the importance of multi-dimensional analytical tools in providing novel insight to the design of the next generation of high-performance devices.

Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides.

Lithium-rich layered transition metal oxide positive electrodes offer access to anion redox at high potentials, thereby promising high energy densities for lithium-ion batteries. However, anion redox is also associated with several unfavorable electrochemical properties, such as open-circuit voltage hysteresis. Here we reveal that in Li1.17-x Ni0.21Co0.08Mn0.54O2, these properties arise from a strong coupling between anion redox and cation migration. We combine various X-ray spectroscopic, microscopic, and structural probes to show that partially reversible transition metal migration decreases the potential of the bulk oxygen redox couple by > 1 V, leading to a reordering in the anionic and cationic redox potentials during cycling. First principles calculations show that this is due to the drastic change in the local oxygen coordination environments associated with the transition metal migration. We propose that this mechanism is involved in stabilizing the oxygen redox couple, which we observe spectroscopically to persist for 500 charge/discharge cycles.

Chemical and Morphological Inhomogeneity of Aluminum Metal and Oxides from Soft X-ray Spectromicroscopy.

Oxygen and aluminum K-edge X-ray absorption spectroscopy (XAS), imaging from a scanning transmission X-ray microscope (STXM), and first-principles calculations were used to probe the composition and morphology of bulk aluminum metal, α- and γ-Al2O3, and several types of aluminum nanoparticles. The imaging results agreed with earlier transmission electron microscopy studies that showed a 2 to 5 nm thick layer of Al2O3 on all the Al surfaces. Spectral interpretations were guided by examination of the calculated transition energies, which agreed well with the spectroscopic measurements. Features observed in the experimental O and Al K-edge XAS were used to determine the chemical structure and phase of the Al2O3 on the aluminum surfaces. For unprotected 18 and 100 nm Al nanoparticles, this analysis revealed an oxide layer that was similar to γ-Al2O3 and comprised of both tetrahedral and octahedral Al coordination sites. For oleic acid-protected Al nanoparticles, only tetrahedral Al oxide coordination sites were observed. The results were correlated to trends in the reactivity of the different materials, which suggests that the structures of different Al2O3 layers have an important role in the accessibility of the underlying Al metal toward further oxidation. Combined, the Al K-edge XAS and STXM results provided detailed chemical information that was not obtained from powder X-ray diffraction or imaging from a transmission electron microscope.

Measuring spectroscopy and magnetism of extracted and intracellular magnetosomes using soft X-ray ptychography.

Characterizing the chemistry and magnetism of magnetotactic bacteria (MTB) is an important aspect of understanding the biomineralization mechanism and function of the chains of magnetosomes (Fe3O4 nanoparticles) found in such species. Images and X-ray absorption spectra (XAS) of magnetosomes extracted from, and magnetosomes in, whole Magnetovibrio blakemorei strain MV-1 cells have been recorded using soft X-ray ptychography at the Fe 2p edge. A spatial resolution of 7 nm is demonstrated. Precursor-like and immature magnetosome phases in a whole MV-1 cell were visualized, and their Fe 2p spectra were measured. Based on these results, a model for the pathway of magnetosome biomineralization for MV-1 is proposed. Fe 2p X-ray magnetic circular dichroism (XMCD) spectra have been derived from ptychography image sequences recorded using left and right circular polarization. The shape of the XAS and XMCD signals in the ptychographic absorption spectra of both sample types is identical to the shape and signals measured with conventional bright-field scanning transmission X-ray microscope. A weaker and inverted XMCD signal was observed in the ptychographic phase spectra of the extracted magnetosomes. The XMCD ptychographic phase spectrum of the intracellular magnetosomes differed from the ptychographic phase spectrum of the extracted magnetosomes. These results demonstrate that spectro-ptychography offers a superior means of characterizing the chemical and magnetic properties of MTB at the individual magnetosome level.

Incorporation of Technetium into Spinel Ferrites.

Technetium (99Tc) is a problematic fission product for the long-term disposal of nuclear waste due to its long half-life, high fission yield, and to the environmental mobility of pertechnetate, the stable species in aerobic environments. One approach to preventing 99Tc contamination is using sufficiently durable waste forms. We report the incorporation of technetium into a family of synthetic spinel ferrites that have environmentally durable natural analogs. A combination of X-ray diffraction, X-ray absorption fine structure spectroscopy, and chemical analysis reveals that Tc(IV) replaces Fe(III) in octahedral sites and illustrates how the resulting charge mismatch is balanced. When a large excess of divalent metal ions is present, the charge is predominantly balanced by substitution of Fe(III) by M(II). When a large excess of divalent metal ions is absent, the charge is largely balanced by creation of vacancies among the Fe(III) sites (maghemitization). In most samples, Tc is present in Tc-rich regions rather than being homogeneously distributed.

Origin and hysteresis of lithium compositional spatiodynamics within battery primary particles.

The kinetics and uniformity of ion insertion reactions at the solid-liquid interface govern the rate capability and lifetime, respectively, of electrochemical devices such as Li-ion batteries. Using an operando x-ray microscopy platform that maps the dynamics of the Li composition and insertion rate in Li(x)FePO4, we found that nanoscale spatial variations in rate and in composition control the lithiation pathway at the subparticle length scale. Specifically, spatial variations in the insertion rate constant lead to the formation of nonuniform domains, and the composition dependence of the rate constant amplifies nonuniformities during delithiation but suppresses them during lithiation, and moreover stabilizes the solid solution during lithiation. This coupling of lithium composition and surface reaction rates controls the kinetics and uniformity during electrochemical ion insertion.

A Macrocyclic Chelator That Selectively Binds Ln4+ over Ln3+ by a Factor of 1029.

A tetravalent cerium macrocyclic complex (CeLK4) was prepared with an octadentate terephthalamide ligand comprised of hard catecholate donors and characterized in the solution state by spectrophotometric titrations and electrochemistry and in the crystal by X-ray diffraction. The solution-state studies showed that L exhibits a remarkably high affinity toward Ce4+, with log ß110 = 61(2) and ΔG = -348 kJ/mol, compared with log ß110 = 32.02(2) for the analogous Pr3+ complex. In addition, L exhibits an unusual preference for forming CeL4- relative to formation of the analogous actinide complex, ThL4-, which has ß110 = 53.7(5). The extreme stabilization of tetravalent cerium relative to its trivalent state is also evidenced by the shift of 1.91 V in the redox potential of the Ce3+/Ce4+ couple of the complex (measured at -0.454 V vs SHE). The unprecedented behavior prompted an electronic structure analysis using L3- and M5,4-edge X-ray absorption near-edge structure (XANES) spectroscopies and configuration interaction calculations, which showed that 4f-orbital bonding in CeLK4 has partial covalent character due to ligand-to-metal charge transfer (LMCT) in the ground state. The experimental results are presented in the context of earlier measurements on tetravalent cerium compounds, indicating that the amount of LMCT for CeLK4 is similar to that observed for [Et4N]2[CeCl6] and CeO2 and significantly less than that for the organometallic sandwich compound cerocene, (C8H8)2Ce. A simple model to rationalize changes in 4f orbital bonding for tri- and tetravalent lanthanide and actinide compounds is also provided.

Extraction of Lanthanide and Actinide Ions from Aqueous Mixtures Using a Carboxylic Acid-Functionalized Porous Aromatic Framework.

Porous aromatic frameworks (PAFs) incorporating a high concentration of acid functional groups possess characteristics that are promising for use in separating lanthanide and actinide metal ions, as required in the treatment of radioactive waste. These materials have been shown to be indefinitely stable to concentrated acids and bases, potentially allowing for multiple adsorption/stripping cycles. Additionally, the PAFs combine exceptional features from MOFs and inorganic/activated carbons giving rise to tunable pore surfaces and maximum chemical stability. Herein, we present a study of the adsorption of selected metal ions, Sr(2+), Fe(3+), Nd(3+), and Am(3+), from aqueous solutions employing a carbon-based porous aromatic framework, BPP-7 (Berkeley Porous Polymer-7). This material displays high metal loading capacities together with excellent adsorption selectivity for neodymium over strontium based on Langmuir adsorption isotherms and ideal adsorbed solution theory (IAST) calculations. Based in part upon X-ray absorption spectroscopy studies, the stronger adsorption of neodymium is attributed to multiple metal ion and binding site interactions resulting from the densely functionalized and highly interpenetrated structure of BPP-7. Recyclability and combustibility experiments demonstrate that multiple adsorption/stripping cycles can be completed with minimal degradation of the polymer adsorption capacity.

Nanobubbles at Hydrophilic Particle-Water Interfaces.

The puzzling persistence of nanobubbles breaks Laplace's law for bubbles, which is of great interest for promising applications in surface processing, H2 and CO2 storage, water treatment, and drug delivery. So far, nanobubbles have mostly been reported on hydrophobic planar substrates with atomic flatness. It remains a challenge to quantify nanobubbles on rough and irregular surfaces because of the lack of a characterization technique that can detect both the nanobubble morphology and chemical composition inside individual nanobubble-like objects. Here, by using synchrotron-based scanning transmission soft X-ray microscopy (STXM) with nanometer resolution, we discern nanoscopic gas bubbles of >25 nm with direct in situ proof of O2 inside the nanobubbles at a hydrophilic particle-water interface under ambient conditions. We find a stable cloud of O2 nanobubbles at the diatomite particle-water interface hours after oxygen aeration and temperature variation. The in situ technique may be useful for many surface nanobubble-related studies such as material preparation and property manipulation, phase equilibrium, nucleation kinetics, and relationships with chemical composition within the confined nanoscale space. The oxygen nanobubble clouds may be important in modifying particle-water interfaces and offering breakthrough technologies for oxygen delivery in sediment and/or deep water environments.

Direct Observation of Localized Radial Oxygen Migration in Functioning Tantalum Oxide Memristors.

Oxygen migration in tantalum oxide, a promising next-generation storage material, is studied using in operando X-ray absorption spectromicroscopy. This approach allows a physical description of the evolution of conduction channel and eventual device failure. The observed ring-like patterns of oxygen concentration are modeled using thermophoretic forces and Fick diffusion, establishing the critical role of temperature-driven oxygen migration.

The nature of chemical bonding in actinide and lanthanide ferrocyanides determined by X-ray absorption spectroscopy and density functional theory.

The electronic properties of actinide cations are of fundamental interest to describe intramolecular interactions and chemical bonding in the context of nuclear waste reprocessing or direct storage. The 5f and 6d orbitals are the first partially or totally vacant states in these elements, and the nature of the actinide ligand bonds is related to their ability to overlap with ligand orbitals. Because of its chemical and orbital selectivities, X-ray absorption spectroscopy (XAS) is an effective probe of actinide species frontier orbitals and for understanding actinide cation reactivity toward chelating ligands. The soft X-ray probes of the light elements provide better resolution than actinide L3-edges to obtain electronic information from the ligand. Thus coupling simulations to experimental soft X-ray spectral measurements and complementary quantum chemical calculations yields quantitative information on chemical bonding. In this study, soft X-ray XAS at the K-edges of C and N, and the L2,3-edges of Fe was used to investigate the electronic structures of the well-known ferrocyanide complexes K4Fe(II)(CN)6, thorium hexacyanoferrate Th(IV)Fe(II)(CN)6, and neodymium hexacyanoferrate KNd(III)Fe(II)(CN)6. The soft X-ray spectra were simulated based on quantum chemical calculations. Our results highlight the orbital overlapping effects and atomic effective charges in the Fe(II)(CN)6 building block. In addition to providing a detailed description of the electronic structure of the ferrocyanide complex (K4Fe(II)(CN)6), the results strongly contribute to confirming the actinide 5f and 6d orbital oddity in comparison to lanthanide 4f and 5d.

Enhanced Nonadiabaticity in Vortex Cores due to the Emergent Hall Effect.

We present a combined theoretical and experimental study, investigating the origin of the enhanced nonadiabaticity of magnetic vortex cores. Scanning transmission x-ray microscopy is used to image the vortex core gyration dynamically to measure the nonadiabaticity with high precision, including a high confidence upper bound. We show theoretically, that the large nonadiabaticity parameter observed experimentally can be explained by the presence of local spin currents arising from a texture induced emergent Hall effect. This study demonstrates that the magnetic damping α and nonadiabaticity parameter ß are very sensitive to the topology of the magnetic textures, resulting in an enhanced ratio (ß/α>1) in magnetic vortex cores or Skyrmions.

Effects of Particle Size, Electronic Connectivity, and Incoherent Nanoscale Domains on the Sequence of Lithiation in LiFePO4 Porous Electrodes.

High-resolution X-ray microscopy is used to investigate the sequence of lithiation in LiFePO4 porous electrodes. For electrodes with homogeneous interparticle electronic connectivity via the carbon black network, the smaller particles lithiate first. For electrodes with heterogeneous connectivity, the better-connected particles preferentially lithiate. Correlative electron and X-ray microscopy also reveal the presence of incoherent nanodomains that lithiate as if they are separate particles.

Theory and X-ray Absorption Spectroscopy for Aluminum Coordination Complexes ­ Al K-Edge Studies of Charge and Bonding in (BDI)Al, (BDI)AlR2, and (BDI)AlX2 Complexes.

Polarized aluminum K-edge X-ray absorption near edge structure (XANES) spectroscopy and first-principles calculations were used to probe electronic structure in a series of (BDI)Al, (BDI)AlX2, and (BDI)AlR2 coordination compounds (X = F, Cl, I; R = H, Me; BDI = 2,6-diisopropylphenyl-ß-diketiminate). Spectral interpretations were guided by examination of the calculated transition energies and polarization-dependent oscillator strengths, which agreed well with the XANES spectroscopy measurements. Pre-edge features were assigned to transitions associated with the Al 3p orbitals involved in metal-ligand bonding. Qualitative trends in Al 1s core energy and valence orbital occupation were established through a systematic comparison of excited states derived from Al 3p orbitals with similar symmetries in a molecular orbital framework. These trends suggested that the higher transition energies observed for (BDI)AlX2 systems with more electronegative X(1-) ligands could be ascribed to a decrease in electron density around the aluminum atom, which causes an increase in the attractive potential of the Al nucleus and concomitant increase in the binding energy of the Al 1s core orbitals. For (BDI)Al and (BDI)AlH2 the experimental Al K-edge XANES spectra and spectra calculated using the eXcited electron and Core-Hole (XCH) approach had nearly identical energies for transitions to final state orbitals of similar composition and symmetry. These results implied that the charge distributions about the aluminum atoms in (BDI)Al and (BDI)AlH2 are similar relative to the (BDI)AlX2 and (BDI)AlMe2 compounds, despite having different formal oxidation states of +1 and +3, respectively. However, (BDI)Al was unique in that it exhibited a low-energy feature that was attributed to transitions into a low-lying p-orbital of b1 symmetry that is localized on Al and orthogonal to the (BDI)Al plane. The presence of this low-energy unoccupied molecular orbital on electron-rich (BDI)Al distinguishes its valence electronic structure from that of the formally trivalent compounds (BDI)AlX2 and (BDI)AlR2. The work shows that Al K-edge XANES spectroscopy can be used to provide valuable insight into electronic structure and reactivity relationships for main-group coordination compounds.

Dependence on Crystal Size of the Nanoscale Chemical Phase Distribution and Fracture in LixFePO4.

The performance of battery electrode materials is strongly affected by inefficiencies in utilization kinetics and cycle life as well as size effects. Observations of phase transformations in these materials with high chemical and spatial resolution can elucidate the relationship between chemical processes and mechanical degradation. Soft X-ray ptychographic microscopy combined with X-ray absorption spectroscopy and electron microscopy creates a powerful suite of tools that we use to assess the chemical and morphological changes in lithium iron phosphate (LiFePO4) micro- and nanocrystals that occur upon delithiation. All sizes of partly delithiated crystals were found to contain two phases with a complex correlation between crystallographic orientation and phase distribution. However, the lattice mismatch between LiFePO4 and FePO4 led to severe fracturing on microcrystals, whereas no mechanical damage was observed in nanoplates, indicating that mechanics are a principal driver in the outstanding electrode performance of LiFePO4 nanoparticles. These results demonstrate the importance of engineering the active electrode material in next generation electrical energy storage systems, which will achieve theoretical limits of energy density and extended stability. This work establishes soft X-ray ptychographic chemical imaging as an essential tool to build comprehensive relationships between mechanics and chemistry that guide this engineering design.

Shifts in oxidation states of cerium oxide nanoparticles detected inside intact hydrated cells and organelles.

Cerium oxide nanoparticles (CNPs) have been shown to induce diverse biological effects, ranging from toxic to beneficial. The beneficial effects have been attributed to the potential antioxidant activity of CNPs via certain redox reactions, depending on their oxidation state or Ce(3+)/Ce(4+) ratio. However, this ratio is strongly dependent on the environment and age of the nanoparticles and it is unclear whether and how the complex intracellular environment impacts this ratio and the possible redox reactions of CNPs. To identify any changes in the oxidation state of CNPs in the intracellular environment and better understand their intracellular reactions, we directly quantified the oxidation states of CNPs outside and inside intact hydrated cells and organelles using correlated scanning transmission x-ray and super resolution fluorescence microscopies. By analyzing hundreds of small CNP aggregates, we detected a shift to a higher Ce(3+)/Ce(4+) ratio in CNPs inside versus outside the cells, indicating a net reduction of CNPs in the intracellular environment. We further found a similar ratio in the cytoplasm and in the lysosomes, indicating that the net reduction occurs earlier in the internalization pathway. Together with oxidative stress and toxicity measurements, our observations identify a net reduction of CNPs in the intracellular environment, which is consistent with their involvement in potentially beneficial oxidation reactions, but also point to interactions that can negatively impact the health of the cells.

Probing the Interfacial Interaction in Layered-Carbon-Stabilized Iron Oxide Nanostructures: A Soft X-ray Spectroscopic Study.

We have stabilized the iron oxide nanoparticles (NPs) of various sizes on layered carbon materials (Fe-oxide/C) that show excellent catalytic performance. From the characterization of X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES), scanning transmission X-ray microscopy (STXM) and X-ray magnetic circular dichroism spectroscopy (XMCD), a strong interfacial interaction in the Fe-oxide/C hybrids has been observed between the small iron oxide NPs and layered carbon in contrast to the weak interaction in the large iron oxide NPs. The interfacial interaction between the NPs and layered carbon is found to link with the improved catalytic performance. In addition, the Fe L-edge XMCD spectra show that the large iron oxide NPs are mainly γ-Fe2O3 with a strong ferromagnetic property, whereas the small iron oxide NPs with strong interfacial interaction are mainly α-Fe2O3 or amorphous Fe2O3 with a nonmagnetic property. The results strongly suggest that the interfacial interaction plays a key role for the catalytic performance, and the experimental findings may provide guidance toward rational design of high-performance catalysts.

Covalency in lanthanides. An X-ray absorption spectroscopy and density functional theory study of LnCl6(x-) (x = 3, 2).

Covalency in Ln-Cl bonds of Oh-LnCl6(x-) (x = 3 for Ln = Ce(III), Nd(III), Sm(III), Eu(III), Gd(III); x = 2 for Ln = Ce(IV)) anions has been investigated, primarily using Cl K-edge X-ray absorption spectroscopy (XAS) and time-dependent density functional theory (TDDFT); however, Ce L3,2-edge and M5,4-edge XAS were also used to characterize CeCl6(x-) (x = 2, 3). The M5,4-edge XAS spectra were modeled using configuration interaction calculations. The results were evaluated as a function of (1) the lanthanide (Ln) metal identity, which was varied across the series from Ce to Gd, and (2) the Ln oxidation state (when practical, i.e., formally Ce(III) and Ce(IV)). Pronounced mixing between the Cl 3p- and Ln 5d-orbitals (t2g* and eg*) was observed. Experimental results indicated that Ln 5d-orbital mixing decreased when moving across the lanthanide series. In contrast, oxidizing Ce(III) to Ce(IV) had little effect on Cl 3p and Ce 5d-orbital mixing. For LnCl6(3-) (formally Ln(III)), the 4f-orbitals participated only marginally in covalent bonding, which was consistent with historical descriptions. Surprisingly, there was a marked increase in Cl 3p- and Ce(IV) 4f-orbital mixing (t1u* + t2u*) in CeCl6(2-). This unexpected 4f- and 5d-orbital participation in covalent bonding is presented in the context of recent studies on both tetravalent transition metal and actinide hexahalides, MCl6(2-) (M = Ti, Zr, Hf, U).