MRS Online Proceedings Library (OPL)

Ab initio x-ray-absorption fine structure (XAFS) and x-ray-absorption near edge structure (XANES) standards are developed for molecules and solids. These standard XAFS spectra are obtained from ab initio XAFS calculations, using an automated code, FEFF, which we have extended to include multiple-scattering (MS) and XANES calculations. Our treatment is based on state-of-the-art curved-wave MS theory. Sample results are presented and compared with experiment. We find that these theoretical standards are comparable with experimental measurements, yielding distance determinations better than 0.02 Å and coordination numbers better than 20%. Our approach also gives a new MS interpretation of the σ* shape-resonances in XANES.

Extended x-ray absorption fine structure (EXAFS) spectroscopy has been used to determine the structural environment local to iron(III) and zirconium(IV) cations in respectively, nanoscale iron oxyhydroxide and nanoscale zirconium oxide powders. The iron oxyhydroxide powder, produced by the modified reverse micelle (MRM) technology, was found to have a short-range structure most similar to that of goethite (α-FeOOH). The short-range structure of the zirconium oxide powder, produced using the rapid thermal decomposition of solutes (RTDS) technology, was found to be a mixture of monoclinic zirconia and cubic zirconia environments.

Diffraction anomalous fine structure (DAFS) is the fine structure in the intensity of an X-ray diffraction peak in the vicinity of an absorption edge. DAFS is measured by monitoring the intensity of a diffraction peak as a function of the incident X-ray energy as it is scanned through an absorption edge. It combines the short range structural sensitivity of X-ray absorption spectroscopy with the long range periodicity of X-ray diffraction, and can provide structural information which is not available from these techniques alone, or in combination. We present a methodology which allows extraction of a specific X-ray absorbance spectrum from the DAFS spectrum, and illustrate it for KMnO4. We also demonstrate the ability of DAFS to separate the contributions to the X-ray absorption spectrum of the tetrahedral and octahedral cobalt sites in the spinel Co3O4.

X-ray Absorption Fine Structure (XAFS) spectroscopy measured at glancing angles of incidence has been used to record the varying environments of Ga and As ion-implanted in amorphous silicon at dose levels of 4 1014 atoms cm-2. Both glow discharge deposited films and silicon wafers previously amorphised by ion beam damage were used and apart from small differences relating to the presence of hydrogen in the former the environments of the same impurity in the two forms of amorphous Si were almost indistinguishable for the same beat treatment. The fact that only glow discharge deposited material is electronically active points to differences in the intrinsic defects in these two forms of a-Si. The local atomic structure of impurities in as-implanted specimens, however, was found to alter significantly with annealing. By 400°C, for instance the coordination numbers of both Ga and As approach 3 Si's indicating very few tetrahedral sites are present in the amorphous state. By 700°C, however, the coordination numbers of either impurity Ga and As have increased and the crystallisation local to the impurities can be detected in the XAFS by the appearance of outer shells of atoms. The fraction of crystalline substitutional sites, however, is only around 33% even though the thermal epitaxial regrowth of the remaining Si matrix is practically complete. Recrystallisation of Si is clearly inhibited in the vicinity of dopants by the persistence of 3-fold sites.

We have investigated, using EXAFS, the local atomic structures of four zirconia polymorphs and their solid solutions with Ca2+, Ga3+, Fe3+, y3+, Gd3+, Ge4+, Ti4+, Ce4+ and Nb5+. Structural information up to 9Å, and in one special case up to 10.9Å, from the absorbing atom has been obtained. The characteristic features of local environments of both the host Zr and the dopant cations, and their variations leading to cation ordering and symmetry evolutions, are elucidated in terms of a dopant size effect and an oxygen vacancy effect. The dynamic aspects of the EXAFS Debye-Waller factor are used to shed light on the connection between phase transition, dopant stabilization, and cation-anion correlation.

Cr2O3 is a common nucleating agent useful for forming ceramics from oxide glasses. In this study we have used a variety of synchrotron radiation techniques to examine the atomic structure, crystallinity and microstructure of a magnesium alumino-silicate glass ceramic whose composition is close to that of the mineral cordierite, Mg2Al4Si5O13. X-ray Absorption spectra on the Cr K-edge have been performed with samples that were heat treated at different temperatures and times to examine the metamorphosis of the nucleating site. This study reveals that Cr3+ is always octahedrally coordinated with oxygen. In addition, the second nearest neighbour environment changes from an amorphous, single shell of Al/Si, but upon crystallisation, develops into a well ordered Al/Mg shell indicative of the formation of a dilute Cr spinel phase. Powder X-ray Diffraction (XRD) patterns, however, reveal that the major phase initially precipitated (˜900°C) is a stuffed quartz and from the lattice parameters that the composition is SiO2 -rich. With prolonged heat treatment a small quantity of a spinel phase emerges whose composition from its lattice parameter is close to MgAl2O4. Combined Small Angle X-ray Scattering (SAXS) and XRD establish that devitrification at these temperatures is associated with particles about 250 Å in size, on average. Energy dispersive powder diffraction patterns were collected in situ to observe changes in crystallinity with temperature and time. From these measurements the stuffed quartz phase identified at 900°C is found to be intermediate, being eventually replaced at higher temperatures by cordierite with a small quantity of spinel.

The structures of C5H5NiNO in a reversible photochemical reaction were studied via EXAFS, FTIR, and optical absorption spectroscopies. A photoexcited intermediate with distinctively different EXAFS, IR, and optical absorption spectra from those of the ground state molecules was generated upon irradiation using 365 nm light at 20K in a 3-methylpentane solution. The reverse reaction was induced by irradiation with 310 nm light. The EXAFS data analysis has shown a 0. 12 Å elongation of the Ni-N bond and the bending of Ni-N-O in the photoexcited intermediate. Several ZINDO calculations were conducted based on the structures obtained from the EXAFS spectroscopy. These calculations reproduced the changes in the optical spectra and the intramolecular electron transfer in C5H5NiNO.

Chemical reference tables state that the standard potential for the reaction of Pt with water, Pt + 2H2O → Pt(OH)2 + 2H+ + 2e−, is 0.98 V, and electrochemical studies propose that this reaction may occur at potentials as low as 0.8 V [1–5]. Using dispersive x-ray absorption finestructure (XAFS) spectroscopy [6], we have directly probed the structural evolution of a Pt catalyst operating in-situ in a polymer electrolyte [7] fuel cell during cyclic voltammetry. The changes in the number of Pt and O nearest-neighbors and the Pt charge demonstrate a close correspondence with features in the voltammogram. Because dispersive XAFS is very sensitive to detecting structural changes, we have been able to detect the presence of chemisorbed oxygen at potentials of 0.6–0.9 V in the anodic sweep. Since double-layer charging is regarded as the only process in this region for bulk Pt [3–5], these results may reflect a limitation of previous (indirect) studies on Pt electrochemistry, or they may indicate that these clusters are different from their bulk metal counterparts. Exploiting the timeresolving capability of dispersive XAFS, we also monitored changes in the Pt charge and the number of O and Pt nearest-neighbors during the electrochemical oxidation and reduction of the Pt clusters in real-time. The results are inconsistent with those expected from the placeexchange mechanism [8–11] for the formation of the surface oxide on bulk Pt electrodes in aqueous solution;

Our current model for understanding these behaviors is that, relative to bulk Pt, unusual types of surface sites play a major role in determining the reactivity of these clusters.

XANES and EXAFS data indicate that chrysocolla does not contain domains of Cu-metal or Cu-oxides as previously hypothesized, but is most similar to dioptase, a chemically homogeneous crystalline Cu-silicate. The first shell around Cu in chrysocolla is similar to that in dioptase, indicating Cu is coordinated by four oxygens. However, the second shell around Cu in chrysocolla is significantly different than the second shell surrounding Cu in dioptase. The larger amplitude second shell for chrysocolla compared with dioptase is found to be primarily due to differences in the Cu-Cu environments between these two materials.

We have studied both the atomic and electronic structures of nitrogenase Mo-Fe proteins using EXAFS and L-edge spectroscopy. The Mo and Fe K-edge EXAFS data show not only 2.37, 2.70, 2.29, and 2.63 Å first shell Mo-S, Mo-Fe, Fe-S, and Fe-Fe distances, but also 3.8, 4.3 and 5.1 Å second shell Fe-Fe, Fe-S, and Mo-Fe interactions. These observed distances are consistent with recent crystallographic models for the M-center. The average Mcenter distances have been defined with unprecedented accuracy. The nitrogenase Fe L-edge spectra have been obtained for the first time, and the result suggests that the Fe exists in both Fe2+ and Fe3+ states.

X-ray absorption spectroscopy has been used to characterize the local structure surrounding Ga and Ge in BaO-Ga2O3-GeO2 glasses. Based on comparisons with results for crystalline standards of known structure, Ge and Ga in all of the glasses are primarily in tetrahedral coordination; no evidence for octahedral coordination is observed. Bond length and coordination values derived from fitting calculations are fairly uniform for all glasses, however disorder increases with increase in BaO and decrease in GeO2 content, especially in the second shell surrounding Ge and Ga. Ge absorption edges for all glasses resemble smoothed BGG-An or GeO2 [quartz] edges. Ga absorption edges of the glasses also resemble the BGG-An edge which has two edge maxima, but in the glasses the relative intensities of the two maxima vary with composition.

Comprehensive studies using X-ray Absorption Fine Structure (XAFS) spectroscopy of the local environments in oxide glasses of alkalis like Na point to the existence of channels of modifying oxide within the glass forming network - a Modified Random Network or MRN. It has been proposed that these are the primary pathways for ionic conduction. Such microsegregation demands a high alkali coordination for non-bridging oxygens. This has now been confirmed from analysis of the O near edge XAFS of silica and sodium disilicate glass. Dielectric relaxation in oxide glasses provides considerable evidence for the cooperative action of alkali ions. The MRN has been used to develop specific microscopic models for ionic transport based on XAFS and incorporating correlated ionic movement. These quantitatively predict the ionic conductivity of silicate and aluminosilicate glasses and the compositional dependence of the activation energy with alkali concentration.

The X-ray Absorption Near Edge Structure (XANES) of single-crystal alkali halide salts have been measured at low temperature (T≈80 K). By employing the electron partial-yield detection technique, spectra of NaF, NaCI, and NaBr were obtained near the sodium K-edge and spectra of LiF, NaF, and KF were obtained near the fluorine K-edge. All spectra showed sharp features at the absorption threshold and broader absorption features extending 50–80 eV above threshold. The high energy resolution of the soft X-rays (ΔE/E ≈ 4000) allowed the detection of previously unobserved fine structure, particularly in the near-edge region. The narrow features below and just above threshold are attributed to core-level excitons. The intense, broader peaks further above threshold are assigned to single-electron scattering resonances. An ab initio multiple-scattering calculation is used to model the latter effect. Contributions from atomic multielectron excitations, estimated by a comparison to the K-edge photoabsorption spectrum of Ne in the gas phase, are found to be very small.

The speciation of radionuclides and toxic metals in wastes subjected to microbial action is important in determining the extent of stabilization in a disposal environment. As part of an ongoing study, we investigated the reduction of uranium by a Clostridium sp. using X-ray absorption near edge spectroscopy (XANES) at the National Synchrotron Light Source (NSLS) and X-ray photoelectron spectroscopy (XPS). XPS analysis of uranyl acetate containing hexavalent uranium exhibited a binding energy of 382.0eV at the U 4f7/2 peak. The sample incubated in the presence of bacteria was shifted to lower binding energy (380.6eV), confirming the reduction of U6+ to U4+ at the bacterial surface. XANES analysis, using an electron yield detector, was performed at the Mv absorption edge (3d--> 5f). The absorption peak energy of the sample exhibited a shift from 3551.1eV to 3550.1eV which is higher than uranium metal (3549.6eV) but lower than U4+ (3550.4eV). This indicates the presence of U3+ which is probably located beneath the surface within the biomass. Anaerobic bacterial treatment of wastes containing uranyl ion can result in the stabilization of uranium.

X-ray Absorption Spectroscopy (XAS) of the Rare Earth (RE) 3d levels yields sharp peaks near the edges as a result of strong, quasi-atomic 3d104fn→3d94fn+1 transitions and these transitions exhibit a wealth of spectroscopic features. The XAS measurements of single crystal REPO4 (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Th, Dy, Er) at the 3d edge were performed in the total yield mode at beam line 8–2 at the Stanford Synchrotron Radiation Laboratory (SSRL). The XAS spectra of the RE ions in the orthophosphate matrix generally resemble the XAS of the corresponding RE metal. This is not unexpected and emphasizes the major contribution of the trivalent state to the electronic transitions at the RE 3d edges. These spectra unequivocally identify the transitions originating from well-characterized RE cores and correlate well with previous theoretical investigations.

Direct comparisons between the surface and bulk of diverse materials can be made by simultaneous electron yield (5 nm depth sensitivity) and fluorescence yield (200 nm) ultra soft x-ray absorption spectroscopy measurements utilizing a rapid sample interchange apparatus. For example the orientations of functional groups have been characterized at and near the surface of a series of model polymeric materials highlighting the chemical and molecular sensitivity of ultra soft x-ray absorption spectroscopy. In addition we discuss a bulk sensitive use of fluorescence yield to non destructively study a buried metal polymer interface. A second bulk sensitive example is the use of fluorescence yield oxygen K near edge x-ray spectroscopy as a method to determine the hole state density of high Tc materials.

Because near edge X-ray absorption fine structures (NEXAFS) are due to dipole transitions from core levels to unoccupied molecular orbitals, it is an ideal technique for obtaining molecular level information about the orientations of ordered species. Such information is important in assessing the relative importance of various intermolecular forces. For example in long chain molecules with bulky functional groups it is not clear a priori whether the attractive van der Waals forces will overcome the repulsive steric interactions to allow for ordering in thin films. To investigate this we have performed C-K edge NEXAFS spectroscopy studies of thin films of the paraffinic molecules C36H74 and (C18H37)2(NCO)C6H4CO2−+NH2(C18H37)2. The films were grown on a Si(111) surface covered with a native oxide. For the normal alkane, the spectra are dominated by three resolvable (Δhυ≈0.5 eV) features: one (C-H)* peak and two σ*(C-C) peaks. The polarization dependence of these peaks indicates that sub-250 Å C36H74 films have the chains of the molecules aligned along the surface normal. For ≈ 2000 Å films the molecular axes are inclined at angle of 32° with respect to the surface normal, in agreement with the crystalline structure determined by X-ray diffraction. In addition to the major structures of the alkane chains, the C-K NEXAFS of (C18H37)2(NCO)C6H4CO2−+NH2(C18H37)2 has a distinct π* peak due to the phenyl ring. Analysis of these resonances indicates that the C-C chains are also aligned along the surface normal, while the phenyl ring is inclined at an angle of 42° with respect to the normal. In conclusion NEXAFS spectroscopy has been used to show that, for the molecules studied, the attractive van der Waals interactions dominate the repulsive forces thus allowing the molecules to orient with their chains aligned along the surface normal in thin films formed on native oxide covered Si(111).

X-ray fluorescence spectra from magnetized iron excited by circularly polarized synchrotron radiation are reported. The Fe L2.3 emission band gives rise to a distinct dichroic signal in confirmation of a recent theoretical prediction. These preliminary experiments carried out with “white” and only partially circularly-polarized synchrotron radiation nevertheless reveal an asymmetry which reflects the spin polarization of the Fe 3d valence electrons. The usefulness of developing specialized high flux circularly-polarized sources is clearly established.

In constrast to other rare earths, replacement of Y with Pr in the high Tc superconductor YBa2Cu3O7 has a pronounced effect on Tc, with Tc going to zero at ≈ 55% Pr. We present Pr XAFS K-edge data for various concentrations of Pr in Y1−xPrxBa2Cu3O7. The Pr K-edge data show that the amplitude of the first Pr-O peak is greatly reduced when compared to the first Y-O peak in pure YBa2Cu3O7. In constrast, for the Y K-edge data in these alloys, there is little if any change in the oxygen environment, while the Cu K-edge data show at most a 10% reduction in the first Cu-0 peak. Fits to the Pr data suggest either that oxygen atoms in the CuO2 planes are missing or that some oxygen atoms about the Pr become highly disordered. Either result could help explain the reduction of Tc with increasing concentration of Pr.

The detection limit for aluminum using total reflection x-ray fluorescence (TXRF) is approximately 100 times lower for a synchrotron source compared to a conventional source. The detection limit for transition metals is approximately 15 to 40 times lower depending on atomic number and energy of the incident radiation.