X-ray Absorption Fine Structure Investigation Research Articles

Near-Edge X-ray Absorption Fine Structure Investigations of Order in Carbon Nanotube-Based Systems

Probing order in nanotube systems is of fundamental importance in devising applications of these tubes in field emission applications as well as for components of composite materials. We use near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to qualitatively and quantitatively study the degree of order and alignment in a wide range of carbon nanotube-based systems, including single-walled carbon nanotube (SWNT) powder, SWNT films, and aligned multiwalled carbon nanotubes. The results are compared to analogous data obtained from a highly ordered pyrolytic graphite (HOPG) sample.

Ion transport and structure in chalcogenide glasses

Ion-conducting chalcogenide glasses have been chosen to illustrate three specificelectrical effects in glasses and their relationship with the glassy structure.Firstly, the variation of conductivity with the mobile cation content wasinvestigated for glasses from the systems Ag2S–GeS2, Ag2S–As2S3 andxAg2S–60GeS–(40 − x)GeS2.Electrical conductivity and field emission scanning electron microscopymeasurements, carried out over an extremely large composition range, show clearlythe role of the macroscopic structure in the electrical properties. Two conductivityregimes are observed and the change from the low conductivity regime to the highconductivity regime in the phase separated glasses Ag2S–GeS2 and Ag2S–As2S3occurs when the regions containing the Ag-rich phase start to connect. Secondly, amixed glass former effect was observed in the glassy system 0.3Li2S–0.7[(1 − x)SiS2–x GeS2] (0 ≤ x ≤ 1),corresponding to an enhancement of the ionic conductivity for the central region 0.5 ≤ x ≤ 0.64.Structural investigations by Raman and small angle x-ray scattering (SAXS) techniquesindicate that the glasses from the limiting composition ranges are homogeneous whileglasses belonging to the central region are phase separated into two compoundswith compositions close to GeS2 and Li2SiS3. Finally, glasses with composition0.5[(1 − x)Rb2S−xAg2S]–0.5GeS2 show clear manifestations of a mixed cation effect. SAXSmeasurements show that the glasses are homogeneous and extended x-rayabsorption fine structure investigations indicate that each mobile cation, Ag+ orRb+, maintains its own specific environment. On the other hand, Raman spectraindicate a non-linear structural change of the glass matrix upon cationmixing with a rearrangement of the local GeS4 tetrahedra to form chains.

Structure and properties of levitated liquid metals

One major advantage of electromagnetic levitation used in our experiments, is its capability to access the metastable state of an undercooled liquid. This paper reports on measurements of surface tension, viscosity, electrical conductivity and density of undercooled liquid metallic liquids such as Au 56Cu 44, Pd 76Cu 6Si 18, Si, Co 80Pd 20 as well as on Extended X-ray Absorption Fine Structure (EXAFS) investigations of Co 80Pd 20. The measurements of surface tension, viscosity and electrical conductivity were performed under microgravity conditions which reduce the strength of the required electromagnetic levitation fields by orders of magnitude and lead to a higher precision.

Microbeam XAFS Investigations on Fluid Inclusions

AbstractIn this paper, we discuss the use of x-ray absorption fine structure (XAFS) techniques for the determination of structure of metal complexes in fluid inclusions heated to elevated temperatures. Analysis of Zn K-edge XAFS spectra measured from a single hypersaline fluid inclusion in quartz shows that the ZnCl42- complex is dominant at all temperatures up to and including 430 °C. The Zn-Cl bond length was found to decrease uniformly with temperature, up to nearly 2% at 430 °C in comparison to the value at 25 °C. The Zn-Cl mean-square relative displacement increased linearly with temperature, from a value of 0.0043 Å2 at 25 °C to 0.0089 Å2 at 430 °C.

Europium-151 Mössbauer spectroscopic and XANES investigation of europium-exchanged Y-zeolite

Eu3+ in ca. 10 wt% europium-exchanged Y-zeolite is partially reduced by treatment in hydrogen at 600°C to Eu2+. The reduction of Eu3+ is more readily achieved in Y-zeolite than in europium(III) oxide. The discrepancy in the extent of reduction as revealed by151Eu Mossbauer spectroscopy and near edge X-ray absorption fine structure (XANES) is associated with any difference in the recoil free fractions of Eu2+ and Eu3+ which may exist at 298 K and the enhanced sensitivity of the XANES to changes in the europium oxidation state.

New opportunities in XAFS investigation in the 1–2 keV region

We report herein the first of a series of K-edge XAFS (x-ray absorption fine structure) spectra of some selected Mg, Al and Si model compounds in the 1–2 keV region using a newly developed YB 66 monochromator. These elements have K-edges in the 1–2 keV region (Mg: 1303 eV; Al: 1559 and Si: 1839 eV), and have hithertofore often been regarded as relatively spectroscopic silent, both because of the difficulties associated with obtaining XAFS data, due to a lack of synchrotron radiation-stable monochromator, and because there are few other readily available spectroscopic probes of local structure for these elements. The present data clearly show that the K-edge spectra are sensitive probes of the chemical nature of these elements. We forsee that XAFS will provide a valuable spectroscopic probe for these and other elements in a variety of technological, biological and environmentally relevant materials.

Extended X-Ray Absorption Fine Structure Investigations at the Silver K-Edge of Ion Exchanged Sodium Silicate Glasses

The present experiments were performed to study the incorporation of silver in sodium silicate glasses by a silver-sodium ion exchange. The extended X-ray absorption fine structure results at the silver K-edge show that the silver ions are surrounded by oxygen in the first coordination sphere. It could be observed structural rearrangements in dependence on both the silver-sodium exchange ratio and the thermal treatment.

X-ray absorption fine-structure investigation of the ionic compounds NaBr, KBr and RbCl: full multiple-scattering analysis

The x-ray absorption near-edge structures (XANESs) of the Br K edge in NaBr and KBr and the Rb K edge in RbCl have been analysed. In spite of the fact that these compounds have the same type of crystal lattice (NaCl type) the published experimental spectra show different structures. We have used a full multiple-scattering method to investigate the conduction band structures of NaBr, KBr and RbCl. The partial densities of Br and Rb p states in the conduction bands were obtained over a large energy interval for both the ground and the excited states. The latter were used to analyse the experimental data. In contrast with our recent study of AgBr, the Br K XANESs in NaBr and KBr and the Rb K XANES in RbCl are found to be determined by the multiple scattering of photoelectrons in a large cluster (30 or more atoms). We also show that, for the energy interval up to 80 eV above the main edge, the single-scattering approximation in the present study seems to be less satisfactory.

X-ray absorption near-edge structure and extended x-ray absorption fine-structure investigation of Pd silicides.

X-ray absorption spectroscopy is used to investigate the near-L-edge structures of Pd in pure Pd and bulk ${\mathrm{Pd}}_{2}$Si and PdSi silicides. Possible many-body effects are suggested to explain the apparent discrepancy between the occurrence of ${L}_{2}$,3 white lines and the 4d hole filling in Pd silicides. The interpretations of the extended fine structures and of the near-edge features are correlated with each other in order to find common support for a dynamical relaxation model for the ${L}_{2}$,3 deep core holes of Pd silicides.