Showing papers on "Extended X-ray absorption fine structure published in 1993"

X-ray-absorption studies of zirconia polymorphs. I. Characteristic local structures.

Abstract: Characteristic extended x-ray-absorption fine structure and x-ray-absorption near-edge structure spectra of zirconia polymorphs (monoclinic, orthorhombic, tetragonal, and cubic) have been identified at 10 K. For the host cations, these spectra are phase dependent but dopant independent because the dopant perturbation is short ranged. In the tetragonal structure, the first Zr-O shell is bifurcated into two tetrahedra, with outer O ions loosely bound and severely distorted. Although the tetragonal Zr cation network is nearly fcc and higher-order Zr-Zr coordinated shells are resolvable at distances up to 9 \AA{}, other evidence points to incoherent-distortion reflecting phase instability. In monoclinic, orthorhombic, and cubic structures, a similar Zr-O polyhedron with sevenfold coordination and a comparable bond length is observed, along with a large distortion or splitting of the next-nearest-neighbor Zr-Zr shell. Progressive departure from the ideal fluorite structure by reducing the Zr-O coordination and dilating or distorting the local Zr-Zr network is seen to be correlated to increased phase stability but not to long-range crystal symmetry.

X-ray-absorption studies of zirconia polymorphs. II. Effect of Y2O3 dopant on ZrO2 structure.

Abstract: Extended x-ray-absorption fine structure and x-ray-absorption near-edge structure spectra at both the Zr and Y K edge have been obtained at 10 K and room temperature for solid ${\mathrm{ZrO}}_{2}$-${\mathrm{Y}}_{2}$${\mathrm{O}}_{3}$ solutions Zr has very different local structures in the tetragonal and cubic solid solutions in terms of the oxygen-bond distance and coordination number In contrast, the Y local structure is nearly the same in all of the solid solutions Charge-compensating oxygen vacancies caused by Y doping are preferentially located next to Zr ions, leaving eightfold oxygen coordination for the Y ions The nearest-neighbor and next-nearest-neighbor distances in these solid solutions do not follow the virtual-crystal approximation with reference to x-ray-diffraction lattice parameter The distortion of the Zr-cation shell in cubic solid solutions is especially severe so that the long-range fluoritelike order implied by global cubic symmetry is not manifested in the local atomic environment

Hydrogen-bond acceptor abilities of tetrachlorometalate(II) complexes in ionic liquids

Abstract: The novel salts [emim]2[CoCl4] and [emim]2[NiCl4](emim = 1-ethyl-3-methylimidazolium cation) have been prepared for use as crystallographic models in an extended X-ray absorption fine structure (EXAFS) study in ambient-temperature ionic liquids. The salts have been characterized both spectroscopically and crystallographically. The crystals are isomorphous, and contain two distinct anions with differing interactions within the extended hydrogen-bonded structures. The implications of these results for the structure of ambient-temperature ionic liquids are discussed.

X-ray absorption spectroscopy of cuprous-thiolate clusters in proteins and model systems

Abstract: Cuprous-thiolate multimetallic clusters exist in a range of different biological molecules for which no structural information exists from X-ray crystallography. Spectroscopic tools such as X-ray absorption spectroscopy have provided the major structural insights into this family of biological molecules. Recent nuclear magnetic resonance data on silver-substituted metallothionein, thought to be analogous with the copper proteins, have suggested the presence of digonal coordination. In order to test this in the copper case, we have examined a series of structurally characterized cuprous-thiolate model compounds, containing different proportions of digonal and trigonal copper sites, using copper K-edge X-ray absorption spectroscopy. The edge spectra, which have been previously used as a probe for the average copper coordination environment in proteins, show little variation between the models, indicating that these are not useful as a probe of coordination environment in the case of cuprous-thiolate clusters (as opposed to isolated metal sites). We show that systematic trends in the average Cu-S bond length from EXAFS curve-fitting analysis can be used to obtain an estimate of the fraction of digonal and trigonal copper sites. This correlation is applied to a series of different proteins containing cuprous-thiolate clusters which are found to contain significant fractions of digonal copper.more » 41 refs., 7 figs., 3 tabs.« less

In-situ x-ray absorption spectroscopy studies of hydride and carbide formation in supported palladium catalysts

Abstract: Extended X-ray absorption fine structure (EXAFS) spectroscopy as used to characterize hydride and carbide phases in supported Pd catalysts. Transmission EXAFS measurements were made at room temperature on 5% Pd/C and 5% Pd/[gamma]-Al[sub 2]O[sub 3] catalysts. Combined EXAFS and transmission electron microscopy (TEM) results indicate that the average Pd particle diameter is 26 [+-] 8 Angstroms (or Pd dispersion is approximately 45%) in both catalysts. Supported Pd particles in air-exposed catalysts were found to be approximately 98% converted to a disordered Pd oxide; the remaining Pd is in metallic cores (average diameter is approximately 6 Angstroms) inside the oxidized Pd particles. Catalysts were reduced in situ and cooled to 25[degrees]C in H[sub 26] (partial pressure of 26 Torr), yielding a hydride phase with a lattice expansion of 2.2 [+-] 0.2%. The stoichiometry of the hydride phase, PdH[sub x] (x to [approximately] 0. 44), is consistent with previous reports of decreased H[sub 2] sorption capacity, relative to bulk Pd, in supported Pd catalysts. Decomposition of the hydride phase yielded metallic Pd particles with a first-shell Pd-Pd coordination number of 9 [+-] 1. Reaction of the reduced 5% Pd/C catalyst with 1% C[sub 2]H[sub 4]/ Ar at 150[degrees]C for 20 min generatesmore » a Pd carbide phase with an average Pd-Pd distance 1.2% larger than that in Pd metal. The PdC[sub x] phase has a maximum carbon content, x to approximately 0.06, about half that of bulk PdC[sub x[prime]] x to approximately 0.13. 44 refs., 10 figs., 6 tabs.« less

Cumulant analysis of the extended x-ray-absorption fine structure of β-AgI

Abstract: The extended x-ray-absorption fine structure (EXAFS) at the K edge of silver has been measured in β-AgI at different temperatures in the range T=23-410 K. Amplitude and phase of the first-shell EXAFS have been separately analysed in terms of cumulant expansion within the photoelectron wave-vector range κ=2.5-16.5 A -1 taking the 23-K spectrum as reference. Tests on EXAFS simulated from excluded-volume-model distributions have been performed to check the convergence of the cumulant series and the correspondence between polynomial coefficients obtained from EXAFS analysis and exact cumulants

Axial oxygen-centered lattice instabilities in YBa2Cu3O7 : an application of the analysis of extended x-ray-absorption fine structure in anharmonic systems

Abstract: Analysis of polarized copper {ital K}-edge extended x-ray-absorption fine-structure (EXAFS) measurements on YBa{sub 2}Cu{sub 3}O{sub 7} for temperatures 10{le}{ital T}{sub nom}{le}105 K indicates that the axial oxygen moves in a double-well potential which softens within a fluctuation region associated with the onset of superconductivity in this material. This fluctuation follows from the coupling between the phonons derived from this double-well potential and the superconducting order parameter. The advantages of EXAFS compared to crystallographic measurements in discerning these aspects of the local structure are discussed. Metrical parameters and the characteristics of the potential are determined by curve fits of the EXAFS. This method is based on the calculation of radial distribution functions from selected model potentials, the forms of which are determined along with the absorber-scatterer distance, scatterer type, and number, by optimization of fits of the experimental data with the EXAFS calculated from these parameters. Unlike perturbative treatments, this approach is ideally suited for highly anharmonic systems because the putative potential can be a very close approximation to the real one and it also offers the advantage of providing dynamical information not available through perturbative treatments.

Temperature dependence of the HNO3 UV absorption cross sections

Abstract: The temperature dependence of the HNO3 absorption cross sections between 240 and 360 K over the wavelength range 195 to 350 nm has been measured using a diode array spectrometer. Absorption cross sections were determined using both (1) absolute pressure measurements at 298 K and (2) a dual absorption cell arrangement in which the absorption spectrum at various temperatures is measured relative to the room temperature absorption spectrum. The HNO3 absorption spectrum showed a temperature dependence which is weak at short wavelengths but stronger at longer wavelengths which are important for photolysis in the lower stratosphere. The 298 K absorption cross sections were found to be larger than the values currently recommended for atmospheric modeling (DeMore et al., 1992). Our absorption cross section data are critically compared with the previous measurements of both room temperature and temperature-dependent absorption cross sections. Temperature-dependent absorption cross sections of HNO3 are recommended for use in atmospheric modeling. These temperature dependent HNO3 absorption cross sections were used in a two-dimensional dynamical-photochemical model to demonstrate the effects of the revised absorption cross sections on loss rate of HNO3 and the abundance of NO2 in the stratosphere.

Re-investigation of titanium silicalite by X-ray absorption spectroscopy : are the novel titanium sites real ?

Abstract: EXAFS studies have been carried out on titanium silicalite-1 (TS-1). The titanium atom is coordinated to four oxygen atoms at the distance of 1.80(1) A. There is no evidence for the presence of 5- and 6-coordinated titanium species or the novel 4-coordinated titanium species involving [TiO x ] units sharing edges with [SiO4] units. The EXAFS spectrum of TS-1 appears to be consistent with the model that titanium resides at the tetrahedral sites of the framework.

Structural studies in lead germanate glasses: EXAFS and vibrational spectroscopy

Abstract: The Raman, IR absorption and EXAFS spectra at the Ge K-edge and Pb LIII-edge of eight lead germanate glasses, with general formula xPbO(1−x)GeO2 with x = 0.20, 0.25, 0.33, 0.40, 0.50, 0.53, 0.56 and 0.60, have been measured. The occurrence of [GeO6] units besides [GeO4] could not be deduced unambiguously from the data. The vibrational and EXAFS data agree with a progressive depolymerization of the network. Starting from all Ge atoms linked to four bridging oxygens in GeO2 (x = 0), the number of tetrahedral units with one or two non-bridging oxygens increases with x. At low content, Pb2+ ions act as modifiers in the germanate structure, but to a lesser extent than an equivalent number of alkaline ions.

Double-electron excitation channels at the Br K edge of HBr and Br2.

Abstract: The x-ray-absorption spectra of gaseous HBr and ${\mathrm{Br}}_{2}$ at the Br K edge have been studied in order to evidence the contribution of double-electron excitation channels to the atomic-absorption background. Evident features due to the simultaneous excitations of 1s4p, 1s4s, 1s3d, and 1s3p electrons, named ${\mathit{KN}}_{2,3}$, ${\mathit{KN}}_{1}$, ${\mathit{KM}}_{4,5}$, and ${\mathit{KM}}_{2,3}$ channels, respectively, have been identified in both spectra. The data analysis takes proper account of the structural signal of the neighboring atom in the molecules calculated including the ${\mathrm{\ensuremath{\chi}}}_{2}$ single-scattering signals and the ${\mathrm{\ensuremath{\chi}}}_{4}$ triple-scattering contribution in the case of ${\mathrm{Br}}_{2}$. Comparisons of energy positions with self-consistent atomic calculations are in excellent agreement with the observed energy onsets. An empirical model function for the double-electron-channel cross section was found to reproduce accurately the observed shapes, the threshold region included. The proper account of both double-electron channels and structural signals allows the correct explanation of the experimental spectra. The fit of the experimental spectra provided a determination of the structural parameters in excellent agreement with the known molecular values. These results are of general importance for structural studies at the Br K edge.

High-Temperature XAS Study of Fe2SiO4 Liquid: Reduced Coordination of Ferrous Iron

Abstract: X-ray absorption spectroscopy (XAS) of Fe(2+) in Fe(2)SiO(4) liquid at 1575 kelvin and 10(-4) gigapascal (1 bar) shows that the Fe(2+) -O bond length is 1.98 +/- 0.02 angstroms compared with approximately 2.22 angstroms in crystalline Fe(2)SiO(4) (fayalite) at the melting point (1478 kelvin), which indicates a decrease in average Fe(2+) coordination number from six in fayalite to four in the liquid. Anharmonicity in the liquid was accounted for using a data analysis procedure. This reduction in coordination number is similar to that observed on the melting of certain ionic salts. These results are used to develop a model of the medium-range structural environment of Fe(2+) in olivine-composition melts, which helps explain some of the properties of Fe(2)SiO(4) liquid, including density, viscosity, and the partitioning of iron and nickel between silicate melts and crystalline olivines. Some of the implications of this model for silicate melts in the Earth's crust and mantle are discussed.

Precipitation of Metallic Platinum into Nafion Ionomer Membranes I . Experimental Results

Abstract: Extended x-ray absorption fine structure (EXAFS) spectroscopy was used to investigate the precipitation of metallic platinum into Nafion ionomer membranes. First, x-ray absorption spectra above the PtL III edge were obtained to compare the structure of platinum tetramine (i) solid, (ii) dissolved in water, and (iii) incorporated to Nafion sheets. The four amine groups remain linked to the platinum atom even after dissolution and incorporation into the membrane. EXAFS collected with dispersive optics was used to follow in situ the incorporation and the precipitation of the salt at PtL III edge. Time dependent concentrations of platinum salt in the membrane were obtained by measuring the time variation of the absorbance of the sample

Reproducibility tests of extended X-ray absorption fine structure for aqua and ammine complexes of first transition metals in solid and aqueous solution

Abstract: X-ray absorption spectra of some aqua and ammine complexes of first transition metals in solid and aqueous solution were measured and analyzed repeatedly. The parameter values of their extended X-ray absorption fine structure (EXAFS) were reproducible within about 0.01 A in the interatomic distance and about 15% in the amplitude factor. Better reproducibilities were obtained for the samples for which the residuals of fitting calculations were small. The "error bar" was also calculated according to the method suggested at the "International Workshop on Standards and Criteria in X-Ray Absorption Spectroscopy, 1988". It was shown that the random errors can be as small as ≤0.001 A in distance and ≤2% in amplitude for samples with simple coordination shell structure.

Nickel K-edge x-ray absorption fine structure of lithium nickel oxides

Abstract: Lithium nickel oxides, Li[sub x]Ni[sub 1[minus]x]O, with the lithium content x varying from 0.40 (close to the maximum value of 0.5) to 0 have been studied by using nickel K-edge X-ray absorption fine structure (XAFS) spectroscopy. The XANES (X-ray absorption near-edge structure) shows several distinct features whose energy and intensity evolve smoothly as a function of x. Quantitative analysis of the EXAFS (extended XAFS) data conclusively showed the presence of systematic structural trends with lithium content, x. In particular, we observed two different Ni-O bond lengths (2.07(1) and 1.94(1) A) with proportions changing with x and systematic variations (with x) of the outer-shell Ni-Ni distances. Overall, the structural trends deduced from the XAFS data were found to be more consistent with an increased nickel oxidation state (Ni[sup +] to Ni[sup +]) with increasing x rather than with an increased oxygen oxidation state (O[sup 2[minus]] to O[sup [minus]]), although the true pictures lies between these two extremes. 37 refs., 11 figs., 4 tabs.

Structure and Nature of the Metal-Support Interface: Characterization of Iridium Clusters on Magnesium Oxide by Extended X-Ray Absorption Fine Structure Spectroscopy.

Abstract: X-ray absorption spectroscopy was used to characterize the metal-support interface in catalysts consisting of very small Ir clusters of nearly uniform nuclearity on the surface of MgO powder. [Ir4(CO)12] on MgO was converted in high yield into [HIr4(CO)11]- and separately into [Ir6(CO)15l2-. Extended X-ray absorption fine structure (EXAFS) data are consistent with the hypothesis that each cluster retained its nuclearity upon decarbonylation. The average Ir-Ir coordination in Ir,/MgO is 2.6 neighbors at a distance of 2.72 A, and the structure is modeled as a mixture of 40-50% tetrahedra and the remainder four-atom rafts on MgO. The average Ir-Ir coordination in Ir6/MgO is 2.7 neighbors at a distance of 2.12 A, and the structure is modeled as a mixture of about 25% Ir6 octahedra and the remainder Ir rafts on MgO. Another sample was treated in H2 at 623 K to give a typical supported metal catalyst having a distribution of Ir clusters averaging 10 8, in diameter. The Ir-support interfaces in Ir4/MgO and Irb/MgO are characterized by single Ir-Mg coordinations at 1.6 A and fourfold Ir-O coordinations at 2.6 A; the interface It atoms are suggested to be atop Mg ions in the (100) faces of MgO. However, this structural model is simplified, and the Ir-Mg interactions are not fully understood. The results demonstrate that the structurally simple supported Ir clusters are among the simplest supported metals; the metal-oxygen distances of approximately 2.6 Aare quite generally characteristic of metal particles and clusters supported on metal oxides and zeolites; the long distance is attributed to metal atoms interacting with oxygen of the support with hydrogen present at the interface.

Temperature dependence of the Pd K-edge extended x-ray-absorption fine structure of PdCx (x~0.13).

Abstract: Pd K-edge extended x-ray-absorption fine-structure (EXAFS) and x-ray-absorption near-edge-structure (XANES) measurements were performed on a Pd carbide phase, ${\mathrm{PdC}}_{\mathit{x}}$ (with x\ensuremath{\sim}0.13), and metallic Pd powder at temperatures between 40 and 423 K. The data were analyzed by nonlinear least-squares fitting using an ab initio theoretical standard. The average lattice expansion of the carbide phase, 2.7\ifmmode\pm\else\textpm\fi{}0.1% relative to Pd metal, agrees well with previous x-ray- and neutron-diffraction results. The temperature dependence of the Pd-Pd nearest-neighbor mean-squared relative displacement (${\mathrm{\ensuremath{\sigma}}}^{2}$) yields a Debye temperature, ${\mathrm{\ensuremath{\Theta}}}_{\mathit{D}}$=295\ifmmode\pm\else\textpm\fi{}10 K for Pd powder. There is an additional static-disorder contribution in ${\mathrm{\ensuremath{\sigma}}}^{2}$ of 0.0009\ifmmode\pm\else\textpm\fi{}0.0002 A${\mathrm{\r{}}}^{2}$, relative to bulk Pd metal. The Debye temperature of ${\mathrm{PdC}}_{0.13}$ powder is also 295\ifmmode\pm\else\textpm\fi{}10 K. There is, however, an additional static-disorder contribution to ${\mathrm{\ensuremath{\sigma}}}^{2}$ of 0.0012 A${\mathrm{\r{}}}^{2}$, relative to Pd powder, probably the result of local distortion of the Pd lattice by interstitial C. The Pd-C coordination is not directly observed in a Fourier transform of the EXAFS. The location of C in octahedral interstices is evident by its effect, through multiple scattering, on the magnitude of the second Pd-Pd peak in the Fourier transform magnitude of the EXAFS data, which is greatly reduced. The Pd K-edge XANES spectrum of ${\mathrm{PdC}}_{0.13}$ is nearly identical to that of Pd metal. There is no significant chemical shift (\ensuremath{\le}0.5 eV) of the Pd K edge of ${\mathrm{PdC}}_{\mathrm{\ensuremath{\sim}}0.13}$.

Ar 2p spectroscopy of free argon clusters

Abstract: Total electron and total and partial ion yield spectra of Ar clusters (with average size up to 600±200) in the region of Ar 2p excitation have been measured using synchrotron radiation and time‐of‐flight mass spectrometry. As the average cluster size increases, the x‐ray absorption spectrum changes systematically from that of atomic Ar to that of solid Ar. The shape of the Ar 2p3/2→4s region is found to be a sensitive monitor of the cluster sizes present in a molecular beam of Ar clusters. Extended x‐ray absorption fine structure (EXAFS) is detected in the spectra of the larger clusters. There is a strong correlation between the intensity of the components of the Ar 2p3/2→4s signal associated with clusters and the intensity of the Fourier filtered first shell Ar 2p EXAFS signal. A low amplitude, high frequency fine structure is observed in the Ar 2p continuum of the heaviest clusters which corresponds closely to that observed in solid Ar. This signal develops with cluster size more slowly than the Ar 2p E...

In-situ x-ray studies of the underpotential deposition of copper on platinum(111)

Abstract: : In-situ x-ray flourescence detected surface EXAFS spectroscopy has been employed to elucidate the in-plane structure of copper underpotentially deposited on a clean and well ordered platinum (111) single crystal electrode. XANES and EXAFS analyses were made as function of applied potential (+0.2V, +0.1V, and 0.0V) corresponding to coverages of approximately 0.5ML, 0.75ML and 1ML, respectively. The charge derived from electrochemical measurements indicate that at the three potentials studied the electrodeposited copper was not completely discharged. The XANES analysis revealed similarities between the electrodeposited copper and the Cu2O reference lending support to earlier findings of a partially discharged copper ad-layer. EXAFS results suggest two cluster phases at the most positive potential (+0.2V) studied and an epitaxial arrangement at the most negative potential (0.0V).

ZnTe at high pressure: X-ray-absorption spectroscopy and x-ray-diffraction studies.

Abstract: ZnTe has been studied by x-ray-absorption spectroscopy at the Zn K edge and by x-ray diffraction up to 30 GPa. Crystallographic transitions were observed at 9.5 and 12 GPa by both techniques. The combination of x-ray-absorption near-edge structure, extended x-ray-absorption fine structure, and diffraction analysis enables us to determine the structure of ZnTe-II which is cinnabar with two pairs of first neighbors at approximately the same distance (\ensuremath{\sim}2.558 \AA{} at 11.7 GPa), maintaining the fourfold coordination, the third pair of neighbors being much farther (\ensuremath{\sim}3.37 \AA{}). At 12 GPa ZnTe-II destabilizes and gives rise to a distorted rocksalt-type coordination, which remains stable at least up to 30 GPa. The succession of transitions is structurally reversible and takes place without amorphization. The hysteresis of the transitions has also been studied. A Murnaghan equation of state fitted to the values of the ZnTe cell parameter in ZnTe-I gives a bulk modulus of 50.5\ifmmode\pm\else\textpm\fi{}3 GPa in good agreement with previous work.

Determination of the zirconium site in zirconium‐substituted Nd2Fe14B

Abstract: We report a direct determination of the Zr site in pseudoternary alloys with a nominal composition of Nd1.6Zr0.4Fe14B using extended x‐ray absorption fine structure (EXAFS). Comparing the Zr EXAFS from these alloys to the Y EXAFS from Y2Fe14B and the Fe EXAFS from Nd2Fe14B reveals that the local Zr and Y environments are nearly identical, and quite disparate from the Fe environment. This finding, and results from x‐ray diffraction and electron microscopy, lead to the conclusion that Zr is incorporated as a substituent for Nd in the Nd2Fe14B structure.

Characterization of the nitrate complexes of Pu(IV) using absorption spectroscopy, {sup 15}N NMR, and EXAFS

Abstract: Nitrate complexes of Pu(IV) are studied in solutions containing nitrate up to 13 molar (M). Three major nitrato complexes are observed and identified using absorption spectroscopy, {sup 15}N nuclear magnetic resonance (NMR), and extended x-ray absorption fine structure (EXAFS) as Pu(NO{sub 3}){sub 2}{sup 2+}, Pu(NO{sub 3}){sub 4}, and Pu(NO{sub 3}){sub 6}{sup 2{minus}}. The possibility that Pu(NO{sub 3}){sub 1}{sup 3+}, Pu(NO{sub 3}){sub 3}{sup 1+} and Pu(NO{sub 3}){sub 5}{sup 1{minus}} are major species in solution is not consistent with these results and an upper limit of 0.10 can be set on the fraction for each of these three nitrate complexes in nitrate containing solutions. Fraction of the three major species in nitric acid over the 1--13 M range were calculated from absorption spectra data. The fraction of Pu(NO{sub 3}){sub 6}{sup 2{minus}} as a function of nitric acid concentration is in good agreement with the literature, whereas the fraction of Pu(NO{sub 3}){sub 2}{sup 2+} and Pu(NO{sub 3}){sub 4} species differ from previous studies. We have modeled the chemical equilibria up to moderate ionic strength ( < 6 molal) using the specific ion interaction theory (SM. Comparison of our experimental observations to literature stability constants that assume the presence of mononitrate species is poor. Stability constant at zero ionic strength for the dinitrato complex is determined to be log({beta}{sub 2}{sup 0})=3.77 {plus_minus} 0.14 (2{sigma}).

Extended x-ray-absorption fine-structure study of GaAsxP1-x semiconducting random solid solutions.

Abstract: The extended x-ray-absorption fine-structure (EXAFS) technique is employed to probe the local atomic structure of the III-V pseudobinary solid solutions GaAsxP1-x as a function of the composition. The first- and second-neighbor bond lengths as well as their variation with the composition are reported. The results imply that the local atomic structure is distorted. Both first-neighbor bond lengths and cation-cation bond lengths exhibit a bimodal structure, showing a tendency to retain the values of pure compounds. Anion-anion bond lengths are much closer to those predicted by Vegard's law; however they still show a significant deviation from it. On the basis of this result, the authors propose that the atomic-scale structure of these compounds can be described by a random distribution of five special coordination tetrahedra. The relation between local atomic structure and EXAFS as well as x-ray-diffraction results is discussed.