XANES Features Research Articles

Probing the electronic nature of Co centers forming the planar ring in octa-nuclear Co complexes using X-ray absorption spectroscopy

The formation of octanuclear cobalt core has been studied in the Co complexes Na[Co8{O2CCH3}8(OCH3)16]NO3(1), Na[Co8{O2CC(CH3)3}8(OCH3)16]NO3(2), Na[Co8{O2CC(CH3)3}8 (OC2H5)16]NO3(3) by analysing the X-ray absorption spectra at the Co K-edge. Analysis of EXAFS confirmed octahedral coordination around the Co atoms. The presence of neighbouring Co atoms has been associated to the octanuclear nature of the complexes. The XANES features have been correlated with the oxidation state of the absorbing Co atoms and their coordination in the complexes. XANES spectra have been simulated by ab-initio XANES calculations performed for the Co centres forming the planar ring in order to investigate and compare the electronic nature of these metal centres. Co density of states (DOS), i.e., s-, p- and d-DOS have also been calculated to explain the different features of XANES spectra. The simulated XANES spectra are further compared with the experimental spectra to probe the effect of different ligands present in the complexes.

Investigating cubane formation and effect of co-crystallization agents in oxo-bridged Co complexes using X-ray absorption spectroscopy

• Cubane core in Co-oxo complexes studied using XAS to probe its spectroscopic nature. • XANES shows presence of octahedrally coordinated Co 3+ ions in cubane structure. • EXAFS confirm octahedral Co with 6 O/N, 3 Co-Co backscattering forming cubane core. • XANES simulation indicate interaction with co-crystallizing salts NaNO 3 and NaClO 4 . • Formation of defect dicubane with Co 2+ /Co 3+ ions predicted by XANES simulation. The formation of cubane core and effect of co-crystallization agent have been studied in the oxo-bridged Co complexes using X-ray absorption spectroscopy. XAFS at Co K-edge have been measured in the oxo-bridged Co complexes [Co 4 O 4 (O 2 CCH 3 ) 4 (C 5 H 5 N) 4 ]( 1 ), 1 .0.5NaNO 3 ·8H 2 O( 2 ), 1 .NaClO 4 ·3·5H 2 O( 3 ) and [Co 4 O 4 (O 2 CCH 3 ) 4 (NH 3 ) 4 ]( 4 ). Complexes 2 and 3 actually contain the same complex 1 co-crystalized by NaNO 3 and NaClO 4 moieties. While 1 and 4 are in powder form, 2 and 3 are in crystalline form. Different XANES features have been correlated to the coordination around the absorbing Co atom in all the complexes. EXAFS analysis has been performed to determine coordination environment around the Co atoms to confirm the formation of the oxo-cubane structure in these complexes. For each of the Co centers present at the four different positions of the cubane core in complexes 2 and 3, XANES spectra have been theoretically simulated and simultaneously p-DOS calculated to examine the effect of co-crystallizing moieties interactions with the individual Co centres present in these two complexes. After preliminary indication by EXAFS analysis, XANES simulations were also performed to examine the presence of defect dicubane structure in 4. The study can be helpful in the detection of these structures during in-situ/operando studies of these catalytically active complexes.

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 .

Looking into the structure of Ge nano-crystals through combined Diffraction/XAFS

Nano-particles of different materials including II-VI (ZnS, ZnO, etc.), III-V (AlN, GaN, etc.) and IV (Si, Ge, etc.) have been applied widely because of their superb optical-electrical properties. More importantly, the size & structure-dependent character of their physical properties due to the quantum confinement effect attracted much research interest during the past decades [1]. Ge is characterized by relatively large Exciton Bohr Radius (≍24nm) [3], which potentially favors the quantum confinement effects for tuning the photoluminescence (PL) properties. Besides, the low toxicity of Ge suggests potential biological applications such as cell imaging and labeling [4]. For the structure characterization of our prepared Ge quantum dots (QDs), we used the in-house X-ray pair distribution function (PDF) machine (X-ray from Ag source with wavelength down to 0.056nm) and Diamond light source synchrotron radiation source (XAFS, XRD, PL, etc.) to characterize the atomic structure. Several other methods such as TEM and SAED were used to provide a comprehensive picture of QDs on nanoscale. The results showed that our as-prepared Ge QDs are amorphous with small size, and annealing process could attenuate the Ge-O bonding. In both aged as prepared and fresh annealed samples we observed a diffraction peak corresponding to d-spacing around 2.8Å, which can be associated with either ST12 or BC8 metastable phase of Ge. Moreover, samples showing extra diffraction peaks also shows different XANES features from that of diamond Ge. The fdmnes calculation and FitIt fitting was carried out to further look into structural information.

X-Ray Absorption Studies of Poly(vinylferrocene) Polymers for Anion Separation

X-ray absorption spectroscopy (XAS) was used to study the electrochemical incorporation of perrhenate anion, from a 0.1 M solution, into poly(vinylferrocene) (PVFc) and a modified PVFc (a copolymer of 30% t-butyl acetylferrocene and 70% t-butyl divinyl ferrocene). The polymers were deposited on a carbon cloth current collector from a solution of the polymers in In situ XAS measurements were done at the Fe K edge on the reduced polymers and at 0.9 V vs. Ag/AgCl. Ex situ XAS was done at the edge after oxidation of the polymers at 0.9 V. The oxidized electrodes were washed in water to remove dissolved in the electrode pores. XAS was done both on wet-washed electrodes and on electrodes that were dried. XAS showed that at 0.9 V the Fe was oxidized from a ferrocene to a ferrocenium moiety and the bond distance increased from 2.05 to 2.08 Å. Both the X-ray absorption near-edge spectroscopy and extended X-ray absorption fine structure (EXAFS) results are consistent with having of the ferrocene moieties in the polymer oxidized at 0.9 V. The Fe K-edge EXAFS showed no direct indication of interaction of Fe with At the edge the only indication of interaction of with the polymers was a slight change in the XANES features. © 2002 The Electrochemical Society. All rights reserved.

Temperature dependent polarized XANES spectra for Zn-doped LSCO system

The cuprates seem to exhibit statistics, dimensionality and phase transitions in novel ways. The nature of excitations (i.e. quasiparticle or collective), spin-charge separation, stripes (static and dynamics), inhomogeneities, pseudogap, effect of impurity dopings (e.g. Zn, Ni) and any other phenomenon in these materials must be consistently understood. Zn-doped LSCO single crystal, were grown by travelling solvent floating zone technique. Temperature dependent polarized XANES (near edge local structure) spectra were measured at the BL13-B1 (Photon Factory) in the fluorescence mode from 10 to 300 K. Since both stripes and nonmagnetic Zn impurities substituted for Cu give rise to inhomogeneous charge and spin distribution it is interesting to understand the interplay of Zn impurities and stripes. To understand these points we have used Zn-doping and some of the results obtained are as follows: The spectra show a strong dependence with respect to the polarization angle, θ, as is evident at any temperature by comparing the spectra where the electric field vector is parallel to the ab-plane to the one where it is parallel to the c-axis. By using the XANES (temperature) difference spectra we have determined T * (experimentally we find, T *≈ 160–170 K) for this sample. The XANES difference spectra shows that the changes in XANES features are larger in the ab-plane than the c-axis, this trend is expected since zinc is doped in the ab-plane at the copper site. Our study also complements the results in literature namely that zinc doping does not affect the c-axis transport.

X-ray absorption studies of high performance Low-k dielectric materials

ABSTRACTThe XAFS measurement of the MSQ type low-k dielectrics (LKD™) was conducted to clarify the structure change with and without the EB cure. Furthermore, three different types of other MSQ films, the ladder structure, the random structure and the CVD film, have been investigated as references. We have determined Si-O-Si bond angle and Si-O(Si-C) bond length by fitting the Fourier transformed EXAFS spectra.The ladder structure and the random structure have Si-O-Si bond angle of 133 and 146, respectively.Si-O-Si bond angle of LKD™ film is among that of the ladder and the random structure, and the XANES spectrum of LKD™ film displays two broad features, corresponding to the mixture of both structures. In contrast, Si-O-Si angles of the EB cured LKD™ film and the CVD film resemble each other, and the XANES features of both films are almost identical with that of the random structure.We have confirmed that the EB cure process for LKD™ film makes drastic structure change from the mixture of ladder and random structure to the random network structure.

An EXAFS study of the local structural environments of Fe, Co, Zn and Mg in natural and synthetic staurolites

AbstractX-ray absorption spectroscopy (XAS) has been used to study the local structures of Fe, Co, Zn, and Mg in synthetic and natural staurolites. Results for the near-edge features (XANES) and refined EXAFS were used to deduce the crystallographic site(s) occupied for each element. The least squares refined, mean first shell metal-O bond lengths, coordination numbers, and Debye-Waller factors are in the ranges: Fe 1.97–1.99 Å, 3.5–4.1, 0.013–0.023 Å2; Co 1.97–1.98 Å, 3.5–3.9, 0.009–0.014 Å2; Zn 1.95–1.96 Å, 3.7–3.8, 0.008–0.009 Å2; and Mn 1.99 Å, 3.8, 0.012 Å2. No significant differences were found which depended on element concentration or whether the sampes are synthetic or natural. The refined bond length for Mg in end-member Mg-staurolite is 2.00 Å but EXAFS data did not allow refinement to give reliable coordination number and Debye-Waller factor estimates. The EXAFS data, together with staurolite and model compound XANES features, suggest that > about 90% of Fe, Co, Zn and Mn are concentrated in the tetrahedral T2 site, while in Mg-staurolite > about 75% of the total Mg is in T2 with the remainder in octahedral sites. In natural staurolite from Pizzo Forno (PF2), a greater proportion of Mg appears to be in octahedral coordination.

M3,2-Edge X-Ray Absorption Near Edge Structure of 5d Metals

We report M 3,2 edge XANES measurements for a series of 5d metals, Hf, Ta, W, Re, Ir, Pt and Au using a total electron yield technique. It was found that the M 3,2 XANES for most 5d metals exhibit intense resonances at the threshold (whitelines). The area under the whiteline correlates with the unoccupied densities of d states of the metal. The M 3,2 XANES features and their systematics are comparable to their L 3,2 counterparts. These results clearly suggest that M 3,2 edge XANES can be used as an alternative probe to the conventional L 3,2 -edge XANES for the unoccupied densities of d states of the 5d metals and their compounds.

Al and Si K absorption edges of Al 2SiO 5 polymorphs using the new YB 66 soft X-ray monochromator

Using a YB 66 monochromator, X-ray absorption spectra of the Al K and Si K absorption edges of the three Al 2SiO 5 polymorphs, sillimanite, andalusite and kyanite, were recorded. For the Al K edges different XANES features could be correlated with the symmetry and the AlO coordination. At the Si K edge all polymorphs show the same white line and only the sillimanite with its special AlOSi arrangement shows a further feature at higher energies.

XAS and XRD Studies on Graphite Intercalation Compounds of Re2O7

Abstract Re2O7 was used as starting material for the preparation of pure low-stage graphite intercalation compounds (GICs). X-ray absorption measurements were carried out at the Re LIII and L1 absorption edges. The EXAFS analysis resulted in a coordination number of four and a Re-O bond length of 173.5 pm for the GICs. In the Re L1 XANES the analysis of the strong pre peak confirmed a slight distorted tetrahedral symmetry. Further XANES features indicated a variation of long-range order for the intercalate. Therefore the XAS measurements determined the tetrahedral perrhenate ion as the intercalated species. The majority of synthesized compounds showed sharp hk0 diffraction patterns. Only some compounds showed first diffuse rings and after several days sharp reflections of an ordered “in-plane” lattice. The analysis of the hk0 diffraction pattern yielded a lattice with the constants a = 652 pm, b = 764 pm, γ = 100°, δ a = 0.6° and δ b = 19.4°.

Structural Studies of Vapour Phase Transport Reactions Using X-ray Absorption Spectroscopy and Matrix Isolation

The vapour above heated ferric chloride (Fe2Cl6) been isolated in nitrogen, argon and neon matrices. The XANES features are indicative of iron in a tetrahedral environment and the interatomic distances from the EXAFS data for the nearest neighbour shells are in good agreement with the vapour phase electron diffraction data. In the case of the nitrogen matrix isolated species the Fe...Fe distance has been determined, indicating the potential usefulness of matrix XAS for the study of vapour phase intermediates. The reasons for the non-observance of this shell in argon and neon matrices are considered.

K XANES studies of 3d transition metal dopants in YBa2(Cu1-xMx)3O7- delta ; x

In continuation of an earlier report on the EXAFS studies (see ibid., vol.4, p.6865 (1992)) the authors present the results of K XANES measurements on the 3d dopants, Mn to Ni and Zn, in the 1-2-3 lattice. It is found, perhaps for the first time, that the Mn dopant in a 1-2-3 lattice is in a divalent state. It is inferred from the K XANES features that Mn occupies the Cu(1) chain site in the 1-2-3 lattice. The results of consolidated K XANES measurements on the valence states of Fe, Co, Ni and Zn dopants are consistent with the earlier reports on these heavy 3d ions in 1-2-3 compounds.