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)32+ 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.
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 d
10
metal-based compounds represent the first examples of transparent and luminescent CP glasses.
Low mechanical pressure on amorphous gold thiolate coordination polymers allows the formation of transparent and red emissive glasses.
Grafting thiol-bearing molecules at the surface of silver nano-particles (AgNPs) is a successful strategy to tune their optical and antibacterial properties. The capping layer generated from ...self-assembly of the ligands at the nanoparticle surface determines the range of possible applications of the resulting material. In particular, direct grafting of the thiol heads to surface Ag(I) can occur, with various hybridizations of the S atom, sp versus sp 3. Alternatively, a passivating Ag 2 S layer can form. We make use of S K-edge X-ray absorption near edge structure (XANES) and synchrotron-based X-ray photoelectron spectroscopy (XPS) to probe the metal−ligand interface in different thiol-capped AgNPs. The use of cryogenic conditions for XAS analyses reveals a peculiar spectral signature for thiolates chemisorbed on the AgNPs surface, unambiguously distinguished from that of Ag 2 S. Ab initio simulations of XANES spectra and XPS analyses are used to predict the grafting mode, suggesting that different ligand architectures promote slightly different proportions of sp/sp 3 sites, and a dramatic variability in the stability of the nanomaterial that can evolve toward either self-assembly or dissolution of the AgNPs.
In the constant race for more efficient Li-ion batteries, extensive research has focused on the design of new, more competitive cathode materials, currently limiting the battery performance. The ...improvement of cathode materials demands the detailed understanding of the complex structural mechanisms at play during battery operation, that is, when Li+ ions are inserted and extracted from the cathode. Moreover, new cathode designs involve more and more disordered/nanosized materials for enhanced Li+ cation diffusion and larger specific surfaces. This trend poses new challenges for the structural investigation methods employed, which mostly rely on the periodic and long-range ordered nature of the compounds under study. This is specially the case of the recently discovered nanostructured Li4Mn2O5 high capacity cathode material, which shows record reversible capacities superior to the state-of-the-art Li-Mn-O electrodes and displays a strongly disordered rock salt-type structure. This last feature, mainly due to its synthetic route involving high energy milling, prevented from reaching a full understanding of the lithium exchange mechanism of particular interest in this 3D framework compound. Here, we demonstrate that a thorough description of such a disordered structure can be achieved by a combination of near-edge X-ray absorption spectroscopy and pair distribution function analysis of neutron and X-ray total scattering data, which ultimately lead to the elucidation of the Li cation diffusion pathways.
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.
A new theoretical approach and computational package, FDMX, for general calculations of X‐ray absorption fine structure (XAFS) is described. This is the first full‐potential package to calculate accurate XAFS spectra across a complete energy range within a single framework, presented with example spectra for elemental Sn, rutile TiO2 and FeO6.
•Theoretical study combined DFT + XANES simulations.•Electronic and morphological structures of Cu20 clusters.•New properties of transition metal clusters.
We present ab initio simulations of X-ray ...Absorption Near-Edge Structure (XANES) spectra, performed on model clusters built by fast simulated annealing and optimized by Density Functional Theory (DFT) minimization. As is known, larger stability of Cu clusters with 20 atoms was found in comparison with those with 19 and 21 atoms. Based on this knowledge, we show the sensitivity of the XANES technique on the number of atoms n, (c.a 20), and on the morphology of the Cun nanoclusters. For this study we used both L3 and K edges and found the former more sensitive. In addition, in the case of the K XANES edge, we carry out the simulations using four different methods, to observe their performance in arrays of a few atoms. Even more, we obtain a good agreement between our results and previous predictions on the HOMO-LUMO gaps for these systems.
Birefringence can contribute to x-ray resonant Bragg diffraction and likely explains recent novel data collected on CuO. We prove these statements using ab initio simulations which reproduce the ...experimental polarization effects quantitatively. We show that an unrotated polarization signal-ruled out in resonant magnetic scattering within the electric dipole approximation-arises from the dynamic change in polarization inside the material. We are able to reproduce all the related behavior with circular polarization and its dependence on the angle of rotation about the Bragg wave vector. We provide a tool to disentangle the various physical origins of the polarization rotation, providing a more complete understanding of the illuminated material.
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.
The cationic distribution is decisive for both the magnetic and electric properties of complex oxides. While it can be easily determined in bulk materials using classical methods such as X‐ray or ...neutron diffraction, difficulties arise for thin films owing to the relatively small amount of material to probe. It is shown here that a full determination of the cationic site distribution in thin films is possible through an optimized processing of resonant elastic X‐ray scattering experiments. The method is illustrated using gallium ferrite Ga2−xFexO3 samples which have been the focus of an increasing number of studies this past decade. They indeed represent an alternative to the, to date, only room‐temperature magnetoelectric compound BiFeO3. The methodology can be applied to determine the element distribution over the various crystallographic sites in any crystallized system.
The determination of the cationic distribution in complex oxide thin films has been shown to be possible through a methodological processing of resonant X‐ray diffraction data with a crystallography‐based approach.
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