X-ray Absorption Spectroscopy has been one of the most powerful tools for probing atomic and molecular structures of materials. However, the measured fine structures in the absorption domain do not have adequate dimensionalities to extract three-dimensional structural information of the material of interest. A technique that allows accurate measurements of atomic fine structure in both the absorption and phase domains will open exciting opportunities in a wide range of fundamental and applied research. In this presentation, we will describe a new technique for determining simultaneously the real and imaginary components of the complex atomic form factor. The technique used Fourier Transform Holography with an extended reference and applicable to both crystalline and amorphous samples. Details of an application of the technique in spectroscopy mode to obtain the X-ray Complex Fine Structure across the copper K-edge will be discussed.

Most crystallographers avoid to perform experiments in the area around an absorption edge of an involved heavy element. Although standard refinement procedures account for dispersion effects, the structure models tend to become unreliable in this range of energies. This originates from the fact that tabulated dispersion values f’ and f” are used. Tables from different sources differ significantly from one another and do not take the chemical environment of atoms in a crystal structure into account at all. In contrast, XAFS spectroscopists are particularly interested in the energy range around absorption edges and derive valuable information from it.

Experiments in our home lab at four different wavelengths (Cu Kα, Cu Kβ, Mo Kα, Ag Kα) and moreover at the European Synchrotron Radiation Facility were carried out to investigate the influence of dispersion in structure models. The presentation reports on the results of an inclusion of dispersion refinement into crystal structure determinations. We observe a good correlation between the absorption spectrum of a given element and the refined dispersion values. Furthermore, the structure model remains unchanged before, after and even at the absorption edge.

95% of XAS research uses measurement of secondary fluorescence photons, which suffers from uncalibrated detector efficiencies and a dominant systematic of self-absorption of the fluorescence photon, which compromises accuracy, analysis and insight. We have developed, coded and implemented a novel self-consistent method to correct for self-absorption seen in high-energy fluorescence X-ray measurements [1, 2]. This method and the resulting software package can be applied to any fluorescence data set.

The complexes considered here, n-pr and i-pr, have been shown to have local metal environments with approximate tetrahedral and square planar coordination geometries using transmission mode XAS [3]. This provides an excellent test of fluorescent multi pixel data and demonstrates the merit of using complimentary techniques to confirm molecular geometries.

A dramatic discrepancy is seen between the spectra from the two measurements. This is due to the self-absorption systematic and also to uncalibrated detector efficiencies in the fluorescence measurement. While the detector efficiency can be corrected for, there is currently no self-consistent method for removing the effect of self-absorption from the spectra. In this work, we predict to high accuracy the magnitude of dispersion and energy functional due to self-absorption. As a result, the dispersion is greatly reduced, and the spectral shape follows the classic XAS trend (Fig. 1). The results presented here demonstrate a dramatic improvement over any previous work in the literature. Our modern theory is of the best quality, allowing our self-absorption correction to be applied to any fluorescence XAS data set and opening up an entire class of experimental investigation.

Figure 1. Ni i-pr SeAFFluX-corrected spectra with a scaled overplot of the transmission XAS spectra.

UiO-66/67/68 metal-organic frameworks (MOFs) show an incredible thermal and chemical stability, which makes them promising materials for catalysis. Functionalization has played an important role in enhancing the material potentialities, by insertion of other metals in the inorganic cornerstones and by functionalization of linkers by additional metals.

This research is aimed to investigate the structure and catalytic properties of series of new UiO-67 metal-organic frameworks functionalized with palladium by in situ and operando X-ray absorption spectroscopy (XAS). The main goals were to determine (i) the key steps of formation of the nanoparticles in UiO-67 pores, (ii) prove the stability of materials upon activation and reaction conditions, and (iii) obtain the structure-reactivity relationships during catalytic hydrogenation of carbon dioxide.

The synthesized materials were studied at the at BM31 beamline of ESRF (Grenoble, France) by simultaneous Pd K-edge XAS and X-ray diffraction (XRD), while the output of the mixture was analysed by online mass spectrometer. XAS was used as the main experimental technique, because of its sensitivity to the local atomic environment around palladium atoms. At the same time, XRD confirmed the stability of UiO-67 crystal structure during the formation of catalytically active species [1].

The samples were activated in situ by heating in a flow of H₂ and He from room temperature to 300 °C and left at this temperature for 30 min to allow the formation of nanoparticles. The activated material was cooled down to 240 °C, and exposed to a reaction mixture (7.5, 2.5, and 10 mL/min of H₂, CO₂, and He, respectively). The reaction was run at 240, 200, and 170 °C under total pressure of 1 and 8 bar, for 2 h under each of the above conditions.

Under reaction conditions, interatomic distances (R_Pd-Pd) systematically increase for all samples, being larger for lower temperatures and high pressures. After cooling down in reaction mixture, the values of R_Pd-Pd ~ 2.81 Å are observed, which are close to that reported for palladium hydrides [2]. However, flushing with helium, gives R_Pd-Pd ~ 2.77 Å which is considerably higher than in metallic palladium. This indicates that apart from palladium hydride, an additional phase is present in the samples. The coordination numbers are close to 10 corresponding to the particle size of 2.6 nm [3]. Within the experimental error, the values are stable and do change during reaction. At the same time, XRD shows a response of UiO-67 crystal structure by increasing the cell parameters with increasing pressure and decreasing temperature, indicative of the adsorption of reactive molecules in MOF.

Summarizing, we showed that under reaction conditions a mixture of palladium hydride and carbide phases is formed during CO₂ hydrogenation in Pd nanoparticles confined inside the pores of UiO-67. The hydride phase is stronger at high pressure and low temperatures, but is removed upon flushing in He, while the carbide one is stable even after flushing.

[1] - Bugaev A. L., Guda A. A., Lomachenko K. A., et al. (2018). Faraday Discuss. 208, 287

[2] - Bugaev A. L., Guda A. A., Lomachenko K. A., et al. (2017) J. Phys. Chem. C 121, 18202

[3] - Kamyshova E. G., et al. Radiat. Phys. Chem. in press, doi: 10.1016/j.radphyschem.2019.02.003.

Measurements of mass attenuation coefficients and X-ray absorption fine structure (XAFS) of zinc selenide (ZnSe) are reported to accuracies typically better than 0.13%. The high accuracy of the results presented here is due to our successful implementation of the X-ray Extended Range Technique (XERT), a relatively new methodology, which can be set up on most synchrotron X-ray beamlines. 561 attenuation coefficients were recorded in the energy range of 6.8 keV to 15 keV that was independently calibrated using powder diffractometry, with measurements concentrated at the zinc and selenium pre-edge, near edge and fine structure absorption edge regions. The removal of systematic effects as well as coherent (Thermal Diffuse) and incoherent (Compton) scattering processes produced very high accuracy values of photoelectrc attenuation which in turn yielded a detailed nanostructural analysis of room temperature ZnSe with full uncertainty propagation. Bond lengths, accurate to 0.003 Å to 0.009 Å, or 0.1% to 0.3%, are plausible and physical. Small variation from a crystalline structure suggests local dynamic motion beyond that of a standard crystal lattice, noting that XAFS is sensitive to dynamic correlated motion. The results obtained in this work are the most accurate to date with comparisons to theoretically determined values of the attenuation showing discrepancies from literature theory of up to 4%, motivating further investigation into the origin of such discrepancies.

Nanostructured LiCoPO₄@UiO-66 were facilely synthesized by MW- assisted solvothermal in one pot-coating at three hours. We introduce a facilely and novel route to enhance the conductivity and the performance of electrochemical properties. The X-ray diffraction pattern of the as-synthesized samples was indexed to a single olivine orthorhombic structure with a Pnma space group, as shown in figure a. TEM analysis exhibited that the particle size of LiCoPO₄ was reduced due to the additive of UiO-66 into precursor solution, and there are small particles of ZrO₂ coated the LiCoPO₄ owing to post-annealing, as exhibited in figure b. The first discharge capacity of the LiCoPO₄@UiO-66 electrode was 146 mAh.g^-1 at 0.05 C in a voltage range of 3.0- 5.27 V, corresponding to approximately 87% of its theoretical capacity (167 mAh/g) as shown in figure d. The X-ray absorption for the as-synthesized samples confirmed the phase is a single and orthorhombic structure with space group Pnma- LiCoPO₄ in agreement with calculated spectra by FDMNES, as shown in figure c. At this point, the aim was to determine the local environment of the Co, upon the lithiation/de-lithiation process, while testing the in-situ cell at the C/5 current rate in the 3-5.27 V voltage range. The principal component analysis (PCA) showed that the LiCoPO₄@UiO-66 has two components implying to the noted degradation referred to the resistance of the electrolyte, as shown in figures e,f.

The development and investigation of smart materials, which present bistability when exposed to external stimuli is a key challenge to material physics and chemistry. Among the various types of these materials, the valence tautomers are compounds which switch between different electronic and spin states and can be used as sensors, signal processors and memory storage [1] since their solid structure does not present substantial rupture during the valence tautomerism (VT) interconversion. The VT has been studied in molecules with a cobalt metal center, nitrogen based ancillary ligands and semiquinone radicals [2-3], and it was observed that it is modulated by the ancillary ligand. For these cobalt complexes, the VT takes place in a reversible fashion [4], in both liquid state and solid state, as single crystals, being possibly dependent on the solid-state arrangement of the complexes and on solvation [5-6]. The VT in such molecules can be induced by temperature as first and second order transitions with a wide range of characteristic T_1/2 according to the ancillary ligand. In the low temperature regime, the VT is also shown to be induced with photo irradiation in multiple wavelengths. Interestingly, it can also be induced with soft and hard X-rays irradiation with high yield of metastable isomers [7-8].

Among the cobalt complexes that display VT, the cobalt 3,5-di-tert-butyl semiquinone pyridine complex is a particularly interesting tautomer, because not only its valence tautomerism can be thermo and photo-induced, but also turned on or off by the presence of solvent molecules in the crystal lattice [5]. It can be crystallized in two different forms, with and without a solvent molecule in the crystal lattice. The first shows no temperature dependence of its magnetic susceptibility, and in the second, the same dependence indicates that only half of the cobalt centers in the unit cell present VT, which we confirmed in X-ray diffraction (XRD) experiments. This, along with results of density functional theory (DFT) calculations, raised an interesting possibility of studying the behavior of particular sites of the crystal separately, utilizing X-ray energies around the cobalt K-edge to understand how each particular site responds to the temperature and how the total VT interconversion takes place within the crystal lattice. In our work we combine the site selectivity of XRD and the characteristic resonant X-ray absorption by cobalt atoms in different oxidation states, in order to spatially map the thermo-induced valence tautomerism within the crystal, and also within the cobalt complexes.