A.M. Flank

Unraveling the atomic structure of biogenic silica: Evidence of the structural association of Al and Si in diatom frustules

We used X-ray absorption spectroscopy at the Al K-edge to investigate the atomic structure of biogenic silica and to assess the effect of Al on its crystal chemistry. Our study provides the first direct evidence for a structural association of Al and Si in biogenic silica. In samples of cultured diatoms, Al is present exclusively in fourfold coordination. The location and relative intensity of X-ray absorption near-edge structure (XANES) features suggests the structural insertion of tetrahedral Al inside the silica framework synthesized by the organism. In diatom samples collected in the marine environment, Al is present in mixed six- and fourfold coordination. The relative intensity of XANES structures indicates the coexistence of structural Al with a clay component, which most likely reflects sample contamination by adhering mineral particles. Extended X-ray absorption fine structure spectroscopy has been used to get Al-O distances in biogenic silica of cultured diatoms, confirming a tetrahedral coordination. Because of its effect on solubility and reaction kinetics of biogenic silica, the structural association between Al and biogenic silica at the stage of biosynthesis has consequences for the use of sedimentary biogenic silica as an indicator of past environmental conditions.

27Al MAS NMR and Aluminum X-ray Absorption Near Edge Structure Study of Imogolite and Allophanes

This paper compares the results of 27Al nuclear magnetic resonance spectroscopy (NMR) and Al-K-edge X-ray Absorption Near Edge Structure (XANES) of natural imogolite and allophanes and some crystalline reference minerals. All soil allophanes studied contain 4-coordinated Al (AlIV). The highest relative proportion of AlIV, 21% of the total Al, was found in Si-rich allophane. This value is close to that found in spring allophanes, which were previously considered to be different from soil allophanes. For a quantitative determination of the AlIV/Altotal ratio, NMR is more reliable than XANES, because of the sensitivity of the chemical shift to low AlIV concentrations, but XANES may be used even if paramagnetic impurities (mostly Fe) are present. Al-K XANES also yields more information than NMR on the local environment of AlVI and especially site multiplicity. AlVI XANES of imogolite and allophanes are similar regardless of the Al/Si ratio. They yield two well-resolved resonances with maxima near 1568 and 1570 eV, which indicates the presence of a unique AlVI site by comparison with crystalline references. The presence of only one AlVI site indicates that imogolite and allophanes have an octahedral sheet with a structure similar to 2/1 dioctahedral phyllosilicates but different from gibbsite or kaolinite, previously considered as structural analogues. The 27AlIV MAS NMR peak maxima of allophanes are between 58.6 and 59.8 ppm, in the range observed for crystalline and amorphous framework alumino-silicates, and less positive than those of sheet silicates, which are typically in the range 65–75 ppm. 27Al-H1 CPMAS NMR spectra suggest that both AlIV and AlVI have Al-O-H linkages.

Environments around Al, Si, and Ca in aluminate and aluminosilicate melts by X-ray absorption spectroscopy at high temperature

Abstract Structural data on silicate, aluminate, and aluminosilicate melts are difficult to measure and understand at high temperature. X-ray absorption spectroscopy (XAS) performed in situ at high temperature has been used to probe the local environment of low-Z elements (Al, Si, and Ca). For fully tetrahedral network glasses, CaAl2Si2O8 (anorthite) and CaAl2O4, the modifications in the Al K-edge spectra with increasing temperature can be attributed to a structural rearrangement of the network or to an increase of fivefold-coordinated Al. For the Ca3Al2O6 composition, where Al is localized in a depolymerized tetrahedral site associated with non-bridging O atoms, XAS spectra at the Al K-edge are barely affected by temperature. Depending on the composition, Ca K-edge spectra investigated in these experiments allow us to follow changes in the distortion of the Ca sites in melts at high temperature. The structural modifications at both short and intermediate range upon melting are well shown by these XAS measurements.

A neutron scattering study of supercooled liquid tellurium

Local order in liquid tellurium has been investigated in both liquid and supercooled state. The nearest-neighbours number has been determined, and its variation vs. temperature is discussed in terms of theoretical predictions. The semiconductor-to-metal transition is shown to occur in a narrow temperature range, mainly in the supercooled-state region.

Full multiple-scattering calculations on silicates and oxides at the Al K edge

We present full multiple-scattering calculations at the aluminium K edge that we compare with experiments for four crystalline silicates and oxide minerals. In the different minerals aluminium atoms are either fourfold or sixfold coordinated to oxygen atoms in Al sites that are poorly symmetric. The calculations are based on different choices of one-electron potentials according to aluminium coordinations and crystallographic structures of the compounds. Hence it is possible to determine how the near-edge spectral features are a sensitive probe of the effective potential seen by the photoelectron in the molecular environment. The purpose of this work is to determine on the one hand the relation between Al K-edge spectral features and the geometrical arrangements around the aluminium sites, and on the other hand the electronic structure of the compounds.

Structural study of various (As−Ge−Se−Te) amorphous chalcogenide glasses using X-ray absorption spectroscopy

The structures of some amorphous chalcogenide glasses existing around the As 17 Ge 23 Se 20 Te 40 compositions have been investigated using EXAFS. Absorption data were obtained at the K edge for As, Se, and Ge and the L m edge for tellurium. Amorphous binary or ternary compounds such as As 2 Se 3 and As 15 Ge 30 Se 55 have also been considered to build the model presented here. It is generally thought that molecular units found in the crystalline phase are retained when the compound is prepared as glass because they are energetically favoured. Our study reveals that if the 8-N rule is respected, heteropolar bonding is, in fact, not always favoured. Particularly for the three systems arsenic atoms show a tendency to clustering, and Te=Te bonds are also found in As-Se-Ge-Te alloy.

Determination of the symmetry of the 3d9L states by polarized CuL3XAS spectra of single crystal YBa2Cu3O≈6.9

Abstract The symmetry of the charge carriers 3d 9 L in the high T c superconductor YBa 2 Cu 3 O 6.9 has been determined by measuring the po larized L 3 x-ray absorption spectra of a single crystal. The intinerant holes of the 3d 9 L configuration formed by oxidation of the divalent Cu compound are found to have only 3d 3z 2 -1 2 character at the Cu site. Therefore the charge carriers induced by doping are not formed in the two dimensional band derived by the Cu 3d x 2 -y 2 but in a band derived by the Cu 3d 3z 2 -1 2 orbital orthogonal to the ab plane of the CuO 2 layers. This result indicates that the ligand holes are in a band involving also the BaO layers and the theories of high T c superconductivity have to take account of a second band with different symmetry, beyond a single band models of the electronic structure.

Structure of β-irradiated glasses studied by X-ray absorption and Raman spectroscopies

Abstract The structural modifications induced by external β-irradiation have been investigated to simulate the ageing processes of nuclear glasses under irradiation. Six glasses in the CaO–Al 2 O 3 –SiO 2 system, from 10 to 68 mol% of SiO 2 content, were irradiated with electrons generated by a Van de Graaf accelerator. The structure of these glasses was studied using Raman and X-ray absorption spectroscopies at the Al K-edge. We observe that a band appears at 1557 cm −1 in the Raman spectra of irradiated glasses, which is assigned to molecular O 2 , and that the average Si–O–Si bond angle decreases. An increase of the polymerization of the network is also detected in the Raman spectra of irradiated samples, which corresponds to a shift of the 1100 cm −1 band towards higher frequencies. This change in network polymerization is confirmed by an increase in the intensity of the main resonance at 1565 eV in the X-ray absorption spectra at the Al K-edge.

The relationship between Al and Si in biogenic silica as determined by PIXE and XAS

Abstract Biogenic silica, one of the major constituents of marine sediments, is a potentially powerful paleoceanographic tool, revealing information on past productivity. Interpreting the sedimentary records of the biogenic silica requires, however, an understanding of its preservation. Dissolution of biogenic silica is controlled by the presence of trace elements such as Al. The work in this paper focuses on the association of Al and Si in biogenic silica. The composition and the atomic structure of cultured and natural diatoms were determined by using PIXE and XAS techniques. This study provides the first evidence for a structural association of Al and Si in biogenic silica.

EXPERIMENTAL EVIDENCE OF ITINERANT Cu 3d9 - OXYGEN HOLE MANY BODY CONFIGURATION IN THE HIGH-TC SUPERCONDUCTOR YBa2Cu3O~7

Cu L3 x-ray absorption near edge structure (XANES) and Cu L3 x-ray photoelectron spectroscopy (XPS) of YBa2Cu3O6.5+x are compared. The breakdown of one-electron picture of its electronic structure is reported. The data are interpreted by mixing of Cu 3d9 and of (where is a hole in the oxygen derived band, ligand hole) many body configuration in the initial state. The localization of Cu 3d9 configuration is indicated by the bare Coulomb interaction Udd~6 eV. The conductivity is assigned to the itinerant configuration. The experimental evidence that the additional oxygen x, giving higher Tc, increases the weight of the configuration is reported. The presence of holes on the oxygen atoms is confirmed by the Ols XPS spectra. The Cu3+(Cu 3d8) configuration is not observed in L3 XANES in agreement with valence band XPS giving the energy of the 3d8 excited state at about 12 eV above the ground state. An energy scheme of the many body configurations in YBa2Cu3O~7 is obtained. These experiments give experimental evidence that the high Tc superconductivity is due to pairing of holes in the oxygen valence band interacting with localized electrons at the Cu sites.