The valence state of Yb ion in YbInAu2 compound at high pressure determined by x-ray diffraction and x-ray absorption near edge structure measurements
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X-ray diffraction patterns and LIII-edge x-ray absorption near edge structure (XANES) spectra of YbInAu2 and LuInAu2 compounds have been measured at high pressure and room temperature using a diamond anvil cell and a synchrotron radiation at SPring-8. YbInAu2 is more compressible at pressures lower than 4GPa than above it; the evaluated bulk modulus by Birch-type equation of state is 54.7GPa which is one-half of that of the LuInAu2 (112.5GPa). The mean valence v¯ of Yb ion in YbInAu2 determined by the XANES measurement is an increasing function of pressure: 2.71(2) at normal pressure and 2.94(2) at 10GPa. The rate of increase in v¯ with pressure is two times larger at pressures below 4GPa than that above 4GPa. However, the v¯ is described by a linear increasing function of the lattice compression: v¯=v¯0+2.9∣ΔV∕V0∣ where v¯0 is 2.71. The extrapolation to the trivalent state gives the critical pressure of 13GPa.Keywords:
XANES
Absorption edge
K-edge
Lattice constant
A low-temperature grafting approach using two CuI molecular precursors ([CuOSi(OtBu)3]4 and [CuOtBu]4) and a high-temperature exchange reaction using CuCl were utilized with a mesoporous silica support (SBA-15) to investigate the effects of catalyst preparation on the nature of copper−support interactions and site speciation. Detailed X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine-structure studies (EXAFS) studies were performed to characterize the nature of the Cu sites and the Cu−support interactions. The freshly prepared materials from the nonaqueous grafting of [CuOSi(OtBu)3]4 (CuOSi/SBA (x.x), where x.x refers to the Cu weight %) exhibit CuI site isolation (by EXAFS and XANES). In contrast, EXAFS and XANES studies of the freshly prepared materials from the nonaqueous grafting of [CuOtBu]4 (CuOtBu/SBA (x.x)) suggest that the Cu−O−Cu linkages of the molecular precursor remain intact upon interacting with the support. Isolated CuI sites are observed as the major species in the freshly prepared material from the high-temperature exchange reaction using CuCl (CuCl/SBA (3.0)) (by XANES and EXAFS). Treatment of the materials under He at 573 K leads to loss of the organic species from the grafted materials (by 1H NMR spectroscopy, thermogravimetric analysis, EA, and IR spectroscopy). EXAFS and XANES studies revealed that CuCl/SBA (3.0) and the CuOSi/SBA (x.x) materials still exhibit up to 95% isolated CuI sites, whereas the CuOtBu/SBA (x.x) materials only exhibit Cu as Cu0 nanoparticles of ca. 7 Å in diameter. After calcination under O2 at 573 K, residual chloride from the high-temperature preparation of CuCl/SBA (3.0) leads to formation of crystalline CuO particles, whereas the CuOSi/SBA (x.x) and CuOtBu/SBA (x.x) materials exhibit more amorphous CuO character after an identical oxidative treatment.
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X-ray absorption spectroscopy
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Fe K-edge XANES (X-ray Absorption Near Edge Structure) and EXAFS (Extended X-ray Absorption Fine Structure) spectra have been used to investigate the local structures of hematite (Fe2O3) powders which were synthesized from local iron stone. The formation of hematite with various calcination temperature (500, 650, 800) °C was observed from XRD data of each powder, and Fe K-edge EXAFS and XANES were performed to sample with calcination temperature 800 °C. The XANES spectra confirmed the oxidation state of the powders. The energy position of the first pre-edge peak at (7113.5 ± 0.5) eV confirm Fe3+. The absorption edges of Fe2O3 powders was 7123.41 eV. The first EXAFS data analysis showed that Fe2O3 powder exhibited nearest-neighbor distances RFe-O = 1.56449 Å and RFe-Fe = 3.01449 Å. The XRD lattice parameter values for the sample is in a fair agreement with the nearest-neighbor distances according to the EXAFS data.
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In functional materials, the local environment around active species that may contain just a few nearest-neighboring atomic shells often changes in response to external conditions. Strong disorder in the local environment poses a challenge to commonly used extended X-ray absorption fine structure (EXAFS) analysis. Furthermore, the dilute concentrations of absorbing atoms, small sample size and the constraints of the experimental setup often limit the utility of EXAFS for structural analysis. X-ray absorption near-edge structure (XANES) has been established as a good alternative method to provide local electronic and geometric information of materials. The pre-edge region in the XANES spectra of metal compounds is a useful but relatively under-utilized resource of information of the chemical composition and structural disorder in nano-materials. This study explores two examples of materials in which the transition metal environment is either relatively symmetric or strongly asymmetric. In the former case, EXAFS results agree with those obtained from the pre-edge XANES analysis, whereas in the latter case they are in a seeming contradiction. The two observations are reconciled by revisiting the limitations of EXAFS in the case of a strong, asymmetric bond length disorder, expected for mixed-valence oxides, and emphasize the utility of the pre-edge XANES analysis for detecting local heterogeneities in structural and compositional motifs.
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Poorly crystalline solids play an important role in many low-temperature geochemical processes, such as trace element speciation and biomineralization. Yet, the structures of many such naturally occurring phases are poorly understood. X-ray absorption spectroscopy is a powerful tool that permits chemically and spatially resolved investigations of poorly crystalline materials. In this study, we compare structural and electronic information derived from different regions of chromium K-edge X-ray absorption spectra for a series of poorly ordered iron(III)-chromium(III)-oxyhydroxides. These phases regularly form after the reduction of Cr(VI) by Fe(II) and often dictate the long-term fate of Cr in the environment. The distinct parts of the X-ray absorption spectrum, namely the pre-edge region, the near edge (XANES) region, and the extended (EXAFS) region, provide complementary information about the local chemical environment of Cr. Analysis of the XANES and EXAFS spectra showed that the structure around Cr in the Cr-poor sample is primarily composed of edge-sharing octahedra, whereas the octahedra in the Cr-rich samples are connected by edge-sharing and cornersharing linkages. The analysis of non-local transitions in the pre-edge spectra indicated the absence of Cr clustering at low Cr substitution. This study demonstrates the advantage of complementary pre-edge, XANES, and EXAFS analysis to deduce information on the medium-range environment around Cr in poorly ordered solids.
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A combined X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) methodology is here presented on a series of partially and fully reduced Au(III) samples. This allows monitoring the relative fraction of Au(III) and Au(0) in the studied samples, displaying a consistent and independent outcome. The strategy followed is based, for the first time, on two structural models that can be fitted simultaneously, and it evaluates the correlation among strongly correlated parameters such as coordination number and the Debye-Waller factor. The results of the present EXAFS and XANES approach can be extended to studies based on X-ray absorption spectroscopy experiments for the in situ monitoring of the formation of gold nanoclusters.
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The L1 and L3 x-ray absorption fine structures from the same element in the same material were compared to illustrate the ‘‘join’’ between the x-ray absorption near edge structure (XANES) and the extended x-ray absorption fine structure (EXAFS). In the XANES region the two spectra differed because of the different selection rules; however, the EXAFS spectra were very similar when adjusted for the phase shift. For the oxide samples investigated the L1,3 spectra were similar for E>20 eV which identifies the demarcation between XANES and EXAFS.
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