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  • 1
    Electronic Resource
    Electronic Resource
    Symmetry dependence of x-ray absorption near-edge structure at the metal K edge of 3d transition metal compounds (2001)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 79 (2001), S. 1918-1920 
    Details
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The pre-edge features in a system with "even" symmetry, apart from quadrupolar transition contribution, are mainly dipolar in character, associated with the existence of unoccupied states made up of mixed cation np with higher-neighboring cation-(n-1)d orbitals, and reflect the density of states due to the medium-range order of the system, while in "odd" symmetry materials these pre-edge features are the result of a transition from the 1s to a final density of states of p symmetry due to an unsymmetrical mixing of the ligand wave functions with the central cation d orbitals. In the latter case, they contain not only the p but also the d base of orbitals, similar to a tetrahedral configuration. These results are validated for Fe as a photoabsorber by comparing x-ray absorption near-edge spectra of Fe2SiO4 (fayalite) and Fe2O3 (hematite) to ab initio full multiple scattering calculations at the Fe K edge, but pertain to all systems containing sixfold-coordinated cations. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1405149
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  • 2
    Electronic Resource
    Electronic Resource
    Experimental and theoretical XANES study of the effects of Fe–Mg solid solution in the enstatite–ferrosilite series (2001)
    Chester : International Union of Crystallography (IUCr)
    Journal of synchrotron radiation 8 (2001), S. 966-968 
    Details
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Synthetic orthopyroxenes in the join enstatite - ferrosilite (Mg2Si2O6-Fe2Si2O6) have been studied by XANES with the aim to interpret the variations found in the spectra and evaluate the effects of the Mg-Fe substitution in the orthopyroxene (OPX) structure. The experimental Fe- and Mg-K edge XANES spectra show variations of the peaks intensity as a function of the chemical composition of the samples along the EN-FS join. In the region of the Fe edge, the intensity ratio between peak A and B (IB/IA) varies linearly (R=0.99) with the Mg/Fe substitution. Multiple scattering calculation have been performed both at the Fe and Mg K-edge, to study the geometrical modifications of the M1 and M2 octahedral sites induced by the Mg-Fe substitutions. The spectra, obtained locating Fe (or Mg) in both M1 and M2 and weighing the contributions using the occupancy data for the M1 and M2 sites determined by XRD, allow to discriminate the effects to the total spectrum due to Fe (Mg) located in each sites separately.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0909049500020665
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  • 3
    Electronic Resource
    Electronic Resource
    Characterization of Local Chemistry and Disorder in Synthetic and Natural α-Al2O3 Materials by X-ray Absorption Near-Edge Structure Spectroscopy (1998)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 31 (1998), S. 890-898 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: X-ray absorption fine spectra at the Al K edge were measured experimentally and calculated theoretically via the multiple-scattering formalism for chemically pure and physically perfect synthetic α-Al2O3 (α-alumina), natural ruby/sapphire (corundum) and a series of artificial corundums produced for technical purposes and used as geochemical standards. The Al K-edge spectra differ, despite the identical coordination (short-range arrangement) assumed by O around Al, and vary slightly in relation to the slightly different chemistries of the materials (substitutional defects) as well as on account of the location taken by foreign atoms in the structural lattices (positional defects). A quantitative treatment of the observed changes is made in terms of short-range distortion of the coordination polyhedron and of medium- to long-range modifications in the overall structure, both induced by substitutions. In some technical corundums, the impurities of admixed β-alumina, where Al is both in fourfold and sixfold coordination, produce another small but detectable effect on Al K edges. Therefore, XAFS spectroscopy proves its potential for both measuring a light element such as Al and detecting minor coordination changes and dilute substitutions (∼1–3 wt% as oxide) of the absorber by other atoms, at least under the favourable conditions occurring in this system.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889898007778
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  • 4
    Electronic Resource
    Electronic Resource
    Symmetry role on the pre-edge X-ray absorption fine structure at the metal K edge (2001)
    Chester : International Union of Crystallography (IUCr)
    Journal of synchrotron radiation 8 (2001), S. 215-217 
    Details
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The preedge features in a system with `even' symmetry, apart from quadrupolar transition contribution, are mainly dipolar in character, associated with the existence of unoccupied states made up of mixed cation-4p with higher-neighboring cation-3d orbitals, and reflect the density of states due to the medium-range order of the system. In `odd' symmetry materials these preedge features are the result of a transition from the 1s to a final density of states of p symmetry due to an unsymmetrical mixing of the ligand wave functions with the central cation 3d orbitals, similar to atetrahedral configuration. These results are validated for Fe as a photoabsorber by comparing XAS spectra of Fe2SiO4 (fayalite) to ab initio full multiple scattering calculations at the Fe K edge, but pertain to all systems containing sixfold-coordinated cations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0909049500016502
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  • 5
    Electronic Resource
    Electronic Resource
    XANES study on the valence transitions in cerium oxide nanoparticles (2001)
    Chester : International Union of Crystallography (IUCr)
    Journal of synchrotron radiation 8 (2001), S. 531-532 
    Details
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The aim of this work is the determination of Ce environment and valence state in cerium oxide nanoparticles prepared by the microemulsion method. X-ray absorption near-edge structure measurements at Ce L3 edge were performed on the nanoparticles as a function of annealing temperature, ranging from 298K to 873K under air condition. The experimental results support the conclusion that Ce ion, in the investigated systems, is in trivalence state when the annealing temperature is below 473K. As the temperature increases up to 623K, the XANES spectrum shows the coexistence of Ce3+ and Ce4+states. When the temperature is higher than 623K, the spectra become identical to that of CeO2 with a distinct double-peak structure, corresponding to the Ce4+ state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0909049500016022
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  • 6
    Electronic Resource
    Electronic Resource
    Oxygen 1s ELNES study of perovskites (Ca,Sr,Ba)TiO3 (2001)
    Chester : International Union of Crystallography (IUCr)
    Journal of synchrotron radiation 8 (2001), S. 934-936 
    Details
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The O K-edge spectra of perovskites XTiO3 with X = Ca, Sr and Ba have been measured using electron energy-loss spectroscopy (EELS) and are analyzed using the ab initio full multiple-scattering (MS) calculations. The near-edge structures arise mainly from covalency by direct and/or indirect interaction between oxygen and metal. The coordination number of the cation and the site symmetry also influence the spectral shape and structures. Comparison of full MS simulations of different clusters around the excited atom allows to interpret and correlate the features present in the spectra of each compound to its specific atomic arrangement and electronic structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0909049500020653
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  • 7
    Electronic Resource
    Electronic Resource
    Theoretical analysis of X-ray absorption near-edge structure in forsterite, Mg2SiO4-Pbnm, and fayalite, Fe2SiO4-Pbnm, at room temperature and extreme conditions (1996)
    Springer
    Physics and chemistry of minerals 23 (1996), S. 193-204 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract We calculated the forsterite Mg K-edge and the fayalite Fe K-edge X-ray absorption spectra both for the M 1 and M 2 sites and for the overall edge by using the one-electron multiple-scattering theory. The validity of the theoretical model is well illustrated by comparison of calculations with experimental data at the Mg K-edge of MgO (periclase) and at the Mg and Fe K-edges spectra of forsterite and fayalite. Starting from these results at room conditions, we calculated the Mg and Fe K-edges X-ray absorption spectra of forsterite and fayalite at low and high temperatures and at high pressures as well. Variations of fine structures occur mostly in the intermediate multiple scattering (IMS) regions and as a result of the applied pressure. In order to demonstrate the capability of XAS to lead to deeper knowledge of structure relevant to Earth's upper mantle we also attempted calcuating the high-P edge for Fe 2+ in low-spin using a different occupation of valence electrons. If a change in spin state really occurs in fayalite, our simple model shows that XAS would evidence it easily even with low resolution.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00220730
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  • 8
    Electronic Resource
    Electronic Resource
    The local structure of Ca-Na pyroxenes. I. XANES study at the Na K-edge (1997)
    Springer
    Physics and chemistry of minerals 24 (1997), S. 500-509 
    Details
    ISSN: 1432-2021
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Abstract  X-ray absorption Na K-edge spectra have been recorded on synthetic endmember jadeite and on a series of natural Ca-Na pyroxenes compositionally straddling the Jd-Di join. The C2/c members of the series are systematically different from the P2/n members. Differences can be interpreted and explained by comparing the experimental spectra with theoretical spectra. These have been calculated by the multiple-scattering formalism from the atomic positional parameters determined by single-crystal X-ray diffraction structure refinement on the same samples. In the full multiple scattering region of the spectra (1075 to 1090 eV) C-pyroxenes exhibit three features which reflect the 6-2 configuration of the O back-scattering atoms around the Na absorber located at the center of the cluster (site M2 of the jadeite structure). P-pyroxenes show more complicated spectra in which at least four (possibly five) features can be recognized; they reflect the two types of configuration (6-2 and 4-2-2) of O around Na in the two independent M2 and M21 eight-fold coordinated sites of the omphacite structure. A weak, sometimes poorly resolved peak at 1079 eV is diagnostic and discriminates C- from P-pyroxenes. The Garnet Ridge C2/c impure jadeite exhibits a spectrum which is intermediate between those of jadeite and omphacite. The Hedin-Lundqvist potential proves best for these insulating materials and allows multiple-scattering calculations agreeing well with experiments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002690050065
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