The Application of HEXS and HERFD XANES for Accurate Structural Characterisation of Actinide Nanomaterials: The Case of ThO2.

The structural characterisation of actinide nanoparticles (NPs) is of primary importance and hard to achieve, especially for non-homogeneous samples with NPs less than 3 nm. By combining high-energy X-ray scattering (HEXS) and high-energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD XANES) analysis, we have characterised for the first time both the short- and medium-range order of ThO2 NPs obtained by chemical precipitation. By using this methodology, a novel insight into the structures of NPs at different stages of their formation has been achieved. The pair distribution function revealed a high concentration of ThO2 small units similar to thorium hexamer clusters mixed with 1 nm ThO2 NPs in the initial steps of formation. Drying the precipitates at around 150 °C promoted the recrystallisation of the smallest units into more thermodynamically stable ThO2 NPs. HERFD XANES analysis at the thorium M4 edge, a direct probe for f states, showed variations that we have correlated with the breakdown of the local symmetry around the thorium atoms, which most likely concerns surface atoms. Together, HEXS and HERFD XANES are a powerful methodology for investigating actinide NPs and their formation mechanism.

The Application of HEXS and HERFD XANES for Accurate Structural Characterisation of Actinide Nanomaterials: The Case of ThO2.

Invited for the cover of this issue is Lucia Amidani and co-workers from the The European Synchrotron, Helmholtz Zentrum Dresden-Rossendorf, Lomonosov Moscow State University, Kurchatov Institute, and the Université Grenoble Alpes. The image depicts the atomic structure of the sample being viewed through "atomic googles", which represent the X-ray techniques used in this work. Read the full text of the article at 10.1002/chem.202003360.

Crystal Orbital Overlap Population and X-ray Absorption Spectroscopy.

We present an extension of an ab initio numerical tool calculating X-ray absorption spectroscopies and crystal orbital overlap populations at the same time. Density functional theory is used to calculate the electronic structure in both occupied (valence to core X-ray emission spectroscopy) and nonoccupied states (i.e., X-ray absorption near the edge structure and X-ray magnetic circular dichroism) and to evaluate the orbital overlap typifying the covalency between neighboring atoms. We show how the different features in the experimental spectra can be correlated to the chemical bonds around the absorbing atoms in several examples including acrylonitrile molecules, rutile TiO2, Li2RuO3 high-energy density cathode, ZnO, and anti-ferromagnetic V2O3.

Ca and S K-edge XANES of CaS calculated by different methods: influence of full potential, core hole and Eu doping.

Ca and S K-edge spectra of CaS are calculated by the full-potential Green's function multiple-scattering method, by the FLAPW method and by the finite-difference method. All three techniques lead to similar spectra. Some differences remain close to the edge, both when comparing different calculations with each other and when comparing the calculations with earlier experimental data. Here it is found that using the full potential does not lead to significant improvement over the atomic spheres approximation and that the effect of the core hole can be limited to the photoabsorbing atom alone. Doping CaS with Eu will not affect the Ca and S K-edge XANES of CaS significantly but may give rise to a pre-edge structure not present for clean CaS.

FDMX: extended X-ray absorption fine structure calculations using the finite difference method.

A new theoretical approach and computational package, FDMX, for general calculations of X-ray absorption fine structure (XAFS) over an extended energy range within a full-potential model is presented. The final-state photoelectron wavefunction is calculated over an energy-dependent spatial mesh, allowing for a complete representation of all scattering paths. The electronic potentials and corresponding wavefunctions are subject to constraints based on physicality and self-consistency, allowing for accurate absorption cross sections in the near-edge region, while higher-energy results are enabled by the implementation of effective Debye-Waller damping and new implementations of second-order lifetime broadening. These include inelastic photoelectron scattering and, for the first time, plasmon excitation coupling. This is the first full-potential package available that can calculate accurate XAFS spectra across a complete energy range within a single framework and without fitted parameters. Example spectra are provided for elemental Sn, rutile TiO2 and the FeO6 octahedron.

Structure, Bonding, and Stability of Mercury Complexes with Thiolate and Thioether Ligands from High-Resolution XANES Spectroscopy and First-Principles Calculations.

We present results obtained from high energy-resolution L3-edge XANES spectroscopy and first-principles calculations for the structure, bonding, and stability of mercury(II) complexes with thiolate and thioether ligands in crystalline compounds, aqueous solution, and macromolecular natural organic matter (NOM). Core-to-valence XANES features that vary in intensity differentiate with unprecedented sensitivity the number and identity of Hg ligands and the geometry of the ligand environment. Post-Hartree-Fock XANES calculations, coupled with natural population analysis, performed on MP2-optimized Hg[(SR)2···(RSR)n] complexes show that the shape, position, and number of electronic transitions observed at high energy-resolution are directly correlated to the Hg and S (l,m)-projected empty densities of states and occupations of the hybridized Hg 6s and 5d valence orbitals. Linear two-coordination, the most common coordination geometry in mercury chemistry, yields a sharp 2p to 6s + 5d electronic transition. This transition varies in intensity for Hg bonded to thiol groups in macromolecular NOM. The intensity variation is explained by contributions from next-nearest, low-charge, thioether-type RSR ligands at 3.0-3.3 Å from Hg. Thus, Hg in NOM has two strong bonds to thiol S and k additional weak Hg···S contacts, or 2 + k coordination. The calculated stabilization energy is -5 kcal/mol per RSR ligand. Detection of distant ligands beyond the first coordination shell requires precise measurement of, and comparison to, spectra of reference compounds as well as accurate calculation of spectra for representative molecular models. The combined experimental and theoretical approaches described here for Hg can be applied to other closed-shell atoms, such as Ag(I) and Au(I). To facilitate further calculation of XANES spectra, experimental data, a new crystallographic structure of a key mercury thioether complex, Cartesian coordinates of the computed models, and examples of input files are provided as Supporting Information .

Monitoring morphology and hydrogen coverage of nanometric Pt/γ-Al2 O3 particles by in†situ HERFD-XANES and quantum simulations.

Platinum nanoclusters highly dispersed on γ-alumina are widely used as heterogeneous catalysts. To understand the chemical interplay between the Pt nanoparticles, the support, and the reductive atmosphere, we performed X-ray absorption near edge structure (XANES) in situ experiments recorded in high energy resolution fluorescence detection (HERFD) mode. Spectra are assigned by comparison with simulated XANES spectra on models obtained by molecular dynamics (DFT-MD). We propose platinum cluster morphologies and quantify the hydrogen coverages compatible with XANES spectra recorded at variable hydrogen pressures and temperatures. Using cutting-edge methodologies to assign XANES spectra, this work gives unequalled atomic insights into the characterization of supported nanoclusters.

Surface science approach to the solid-liquid interface: surface-dependent precipitation of Ni(OH)2 on α-Al2O3 surfaces.

Surface-dependent precipitation: The adsorption of Ni(II) complexes in aqueous solution on (0001) and (1102) α-Al(2)O(3) single-crystal surfaces has been studied (see the X-ray absorption spectra obtained for parallel and perpendicular polarization directions). The use of planar model systems emphasizes the crucial role of the Al(2)O(3) orientation for Ni dispersion with practical implications in catalyst preparation procedures.

Transparent and luminescent glasses of gold thiolate coordination polymers.

Obtaining transparent glasses made of functional coordination polymers (CPs) represents a tremendous opportunity for optical applications. In this context, the first transparent and red-emissive glasses of gold thiolate CPs have been obtained by simply applying mechanical pressure to amorphous powders of CPs. The three gold-based CP glasses are composed of either thiophenolate [Au(SPh)] n , phenylmethanethiolate [Au(SMePh)] n or phenylethanethiolate [Au(SEtPh)] n . The presence of a longer alkyl chain between the thiolate and the phenyl ring led to the formation of glass with higher transparency. The glass transitions, measured by thermomechanical analysis (TMA), occurred at lower temperature for CPs with longer alkyl chains. In addition, all three gold thiolate glasses exhibit red emission at 93 K and one of them, [Au(SMePh)] n , remains luminescent even at room temperature. An in-depth structural study of the amorphous gold thiolates by XRD, PDF and EXAFS analysis showed that they are formed of disordered doubly interpenetrated helical chains. These d10 metal-based compounds represent the first examples of transparent and luminescent CP glasses.

Resonant nanodiffraction x-ray imaging reveals role of magnetic domains in complex oxide spin caloritronics.

Spin electronic devices based on crystalline oxide layers with nanoscale thicknesses involve complex structural and magnetic phenomena, including magnetic domains and the coupling of the magnetism to elastic and plastic crystallographic distortion. The magnetism of buried nanoscale layers has a substantial impact on spincaloritronic devices incorporating garnets and other oxides exhibiting the spin Seebeck effect (SSE). Synchrotron hard x-ray nanobeam diffraction techniques combine structural, elemental, and magnetic sensitivity and allow the magnetic domain configuration and structural distortion to be probed in buried layers simultaneously. Resonant scattering at the Gd L2 edge of Gd3Fe5O12 layers yields magnetic contrast with both linear and circular incident x-ray polarization. Domain patterns facet to form low-energy domain wall orientations but also are coupled to elastic features linked to epitaxial growth. Nanobeam magnetic diffraction images reveal diverse magnetic microstructure within emerging SSE materials and a strong coupling of the magnetism to crystallographic distortion.

Epitaxial growth and structure of cobalt ferrite thin films with large inversion parameter on Ag(001).

Cobalt ferrite ultrathin films with the inverse spinel structure are among the best candidates for spin filtering at room temperature. High-quality epitaxial CoFe2O4 films about 4 nm thick have been fabricated on Ag(001) following a three-step method: an ultrathin metallic CoFe2 alloy was first grown in coherent epitaxy on the substrate and then treated twice with O2, first at room temperature and then during annealing. The epitaxial orientation and the surface, interface and film structure were resolved using a combination of low-energy electron diffraction, scanning tunnelling microscopy, Auger electron spectroscopy and in situ grazing-incidence X-ray diffraction. A slight tetragonal distortion was observed, which should drive the easy magnetization axis in-plane due to the large magneto-elastic coupling of such a material. The so-called inversion parameter, i.e. the Co fraction occupying octahedral sites in the ferrite spinel structure, is a key element for its spin-dependent electronic gap. It was obtained through in situ resonant X-ray diffraction measurements collected at both the Co and Fe K edges. The data analysis was performed using FDMNES, an ab initio program already extensively used to simulate X-ray absorption spectroscopy, and shows that the Co ions are predominantly located on octahedral sites with an inversion parameter of 0.88 (5). Ex situ X-ray photoelectron spectroscopy gives an estimation in accordance with the values obtained through diffraction analysis.

Simulation of Surface Resonant X-ray Diffraction.

We present an ab initio numerical tool to simulate surface resonant X-ray diffraction experiments. The crystal truncation rods and the spectra around a given X-ray absorption edge are calculated at any position of the reciprocal space. Density functional theory is used to determine the resonant scattering factor of an atom within its local environment and to calculate the diffraction peak intensities for surfaces covered with a thin film or with one or several adsorbed layers. Besides the sample geometry, the collected data also depend on several parameters, such as beam polarization and incidence and exit angles. In order to account for these factors, a numerical diffractometer mimicking the experimental operation modes has been created. Finally two case studies are presented in order to compare our simulations with experimental spectra: (i) a magnetite thin film deposited on a silver substrate and (ii) an electrochemical interface consisting of bromine atoms adsorbed on copper.

Full-Potential Simulation of X-ray Raman Scattering Spectroscopy.

We present a new ab initio way to calculate X-ray Raman scattering spectra within the independent electron approximation. Our approach avoids any approximation about the shape of the used potential and leads to good agreement between experiment and theory. We show that the momentum transfer dependence in two typical cases, the F K-edge in LiF and the B and N K-edges in hexagonal BN, is well-reproduced. A more in-depth analysis of the electronic states and of the local atomic structure around the absorbing atoms is at hand.

Core-shell-like Au sub-nanometer clusters in Er-implanted silica.

The very early steps of Au metal cluster formation in Er-doped silica have been investigated by high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS). A combined analysis of the near-edge and extended part of the experimental spectra shows that Au cluster nucleation starts from a few Au and O atoms covalently interconnected, likely in the presence of embryonic Au-Au correlation. The first Au clusters, characterized by a well defined Au-Au coordination distance, form upon 400 °C inert annealing. The estimated upper limit of the Gibbs free energy for the associated heterogeneous nucleation is 0.06 eV per atom, suggesting that the Au nucleation is assisted by matrix defects, most likely non-bridging oxygen atoms. The experimental results indicate that the formed subnanometer Au clusters can be applied as effective core-shell systems in which the Au atoms of the 'core' develop a metallic character, whereas the Au atoms in the 'shell' can retain a partially covalent bond with O atoms of the silica matrix. High structural disorder at the Au site is found upon neutral annealing at a moderate temperature (600 °C), likely driven by the configurational disorder of the defective silica matrix. A suitable choice of the Au concentration and annealing temperature allows tailoring of the Au cluster size in the sub-nanometer range. The interaction of the Au cluster surface with the surrounding silica matrix is likely responsible for the infrared luminescence previously reported on the same systems.

Optimized Finite Difference Method for the Full-Potential XANES Simulations: Application to Molecular Adsorption Geometries in MOFs and Metal-Ligand Intersystem Crossing Transients.

Accurate modeling of the X-ray absorption near-edge spectra (XANES) is required to unravel the local structure of metal sites in complex systems and their structural changes upon chemical or light stimuli. Two relevant examples are reported here concerning the following: (i) the effect of molecular adsorption on 3d metals hosted inside metal-organic frameworks and (ii) light induced dynamics of spin crossover in metal-organic complexes. In both cases, the amount of structural models for simulation can reach a hundred, depending on the number of structural parameters. Thus, the choice of an accurate but computationally demanding finite difference method for the ab initio X-ray absorption simulations severely restricts the range of molecular systems that can be analyzed by personal computers. Employing the FDMNES code [Phys. Rev. B, 2001, 63, 125120] we show that this problem can be handled if a proper diagonalization scheme is applied. Due to the use of dedicated solvers for sparse matrices, the calculation time was reduced by more than 1 order of magnitude compared to the standard Gaussian method, while the amount of required RAM was halved. Ni K-edge XANES simulations performed by the accelerated version of the code allowed analyzing the coordination geometry of CO and NO on the Ni active sites in CPO-27-Ni MOF. The Ni-CO configuration was found to be linear, while Ni-NO was bent by almost 90°. Modeling of the Fe K-edge XANES of photoexcited aqueous [Fe(bpy)3](2+) with a 100 ps delay we identified the Fe-N distance elongation and bipyridine rotation upon transition from the initial low-spin to the final high-spin state. Subsequently, the X-ray absorption spectrum for the intermediate triplet state with expected 100 fs lifetime was theoretically predicted.

Calculating X-ray absorption near-edge structure at very low energy.

The measurement and calculation of X-ray absorption near-edge structure at very low energy can provide important information about a metallic center in biological compounds. A rapid overview of the biological applications of this technique is given, then a new method of calculating the spectra is presented. This technique, based on the use of the finite-difference method to solve the Schrödinger equation, is especially precise and potentially applicable to metalloproteins. Examples of its use on an oxide and an organic compound illustrate the kind of spectroscopic information that can be obtained.

Structural and electronic evolution of the As(OH)3 molecule in high temperature aqueous solutions: an x-ray absorption investigation.

The geometrical and electronic structure of the arsenious acid molecule As(OH)(3) in aqueous solutions has been investigated by x-ray absorption spectroscopy (XAS) within extended x-ray absorption spectroscopy (EXAFS) and x-ray absorption near edge structure (XANES), using realistic first-principle calculations in the latter case. This investigation was performed on aqueous solutions of arsenious acid from ambient to supercritical conditions (P = 250 and 600 bars, T