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  • Articles  (117)
  • International Union of Crystallography (IUCr)
  • 2000-2004  (117)
  • 1
    Electronic Resource
    Electronic Resource
    X-ray absorption studies of carbon-related materials (2001)
    Chester : International Union of Crystallography (IUCr)
    Journal of synchrotron radiation 8 (2001), S. 145-149 
    Details
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: X-ray absorption near-edge structure (XANES) measurements have been performed on nitrogen-doped diamond films with three different dopant concentrations and iron-layer-stabilized carbon nanotube (CNT) structures with various diameters at the C K-absorption edge using the sample drain current mode. The C K-edge XANES spectra of these N-doped diamond films resemble that of the undoped diamond regardless of the dopant concentration, which suggest that the overall bonding configuration of the C atom is unaltered. N dopants are found to reduce the intensities of both the sp2- and sp3-bond-derived resonance features in the XANES spectra. On the other hand, the C K-edge XANES spectra of CNTs indicate that the intensities of the π* and σ* bands and the interlayer-state features vary with the diameter of the CNT. This phenomenon may be caused by the Fe-layer-catalysed bending of the graphite sheet and the interaction between C and Fe atoms.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0909049500019750
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  • 2
    Electronic Resource
    Electronic Resource
    Performance of bent-crystal monochromators for high-energy synchrotron radiation (2000)
    Chester : International Union of Crystallography (IUCr)
    Journal of synchrotron radiation 7 (2000), S. 69-77 
    Details
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Two monochromators for high-energy synchrotron radiation have been studied at the elliptical multipole wiggler beamline BL08W of SPring-8. Both monochromator crystals are bent and indirectly water-cooled. In the 100 keV monochromator an efficient cooling scheme is employed. A monochromatic beam is successfully focused using new benders for the 100 keV and 300 keV monochromators. Measured radii of curvature of the bent crystal agree qualitatively with calculation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S090904959901691X
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  • 3
    Electronic Resource
    Electronic Resource
    Crystallization and preliminary X-ray analysis of Citrobacter amalonaticus methylaspartate ammonia lyase (2001)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 57 (2001), S. 1922-1924 
    Details
    ISSN: 1399-0047
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Methylaspartate ammonia lyase (MAL) catalyses the reversible α,β-elimination of ammonia from L-threo-(2S,3S)-3-methylaspartic acid to give mesaconic acid. Crystals of Citrobacter amalonaticus MAL have been obtained by the hanging-drop method of vapour diffusion using ammonium sulfate as the precipitant. Three crystal forms were obtained from identical crystallization conditions, two of which (forms A and B) diffract to high resolution, whilst the third form diffracted poorly. Crystals of form A diffract to beyond 2.1 Å and have been characterized as belonging to one of the enantiomorphic space groups P4122 or P4322, with unit-cell parameters a = b = 66.0, c = 233.1 Å, α = β = γ = 90° and a monomer in the asymmetric unit. Crystals of form B diffract to beyond 1.5 Å and belong to space group C222, with unit-cell parameters a = 128.3, b = 237.4, c = 65.8 Å, α = β = γ = 90° and a dimer in the asymmetric unit. Determination of the structure of MAL will be an important step in resolving current conflicts concerning the enzyme mechanism which differ between one which places MAL as a member of the superfamily of ammonia lyases whose catalytic activity requires a cofactor formed by post-translational modification of the enzyme and another which links MAL to the enolase superfamily.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0907444901016675
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  • 4
    Electronic Resource
    Electronic Resource
    Studies of impurities in magnetic semiconductors: an example of important XAFS applications (2001)
    Chester : International Union of Crystallography (IUCr)
    Journal of synchrotron radiation 8 (2001), S. 874-876 
    Details
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The X-ray absorption fine structure (XAFS) technique has been employed to investigate the local structure and valency about Mn and Fe ions in the III-V diluted magnetic semiconductors In1-xMnxAs and Ga1-xFexAs, prepared by molecular-beam-epitaxy under various growth conditions. These new systems are promising magnetic materials of considerable current interest and with important technical applications including photo-carrier induced magnetism and spin-polarized current devices. The local structure around the magnetic ions can play a pivotal role in affecting the magnetic properties of these semiconductors. Local structure information obtained from XAFS has provided the first direct evidence that the magnetic impurities can indeed substitute for the cation host atoms in samples prepared under appropriate conditions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0909049500016708
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  • 5
    Electronic Resource
    Electronic Resource
    Synchrotron radiation X-ray powder diffractometer with a cylindrical imaging plate (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 1241-1245 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: A synchrotron radiation X-ray powder diffractometer for samples of very small amount has been developed to collect high-quality diffraction patterns under extreme conditions, i.e. at low temperature and/or high pressure. A new cylindrical imaging plate (CIP) is used as a detector, in addition to a conventional flat-type imaging plate (FIP). By using the CIP system, the diffraction data in a diffraction angle range −44 ≤ 2θ ≤ 122° are collected with a dynamic range of about 106. The alignment of the diffractometer, measurement and analysis are automatically operated by a workstation. A performance test shows that the CIP system has spatial resolution of about 0.07° with a dynamic range of 106. The diffraction pattern of a standard sample of Si measured by the CIP system has high quality; the refinement of the structure reaches Rw = 3.68% even in the case of a small amount of sample (about 2 µg) and a short exposure time (60 s). Examples of experiments at low temperatures under ambient and high pressures are also presented.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889800009286
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  • 6
    Electronic Resource
    Electronic Resource
    Elimination of extrinsic components overlapping lattice distortion variations of a silicon single crystal obtained by double-crystal X-ray topography (2000)
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 33 (2000), S. 226-233 
    Details
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: Variations in lattice plane spacing and lattice plane orientation, i.e. Δd/d and Δα, reflect variations in lattice distortion in a single crystal. Double-crystal X-ray topography (DCT) using synchrotron radiation can be used to measure Δd/d and Δα of a silicon single crystal. However, both Δd/d and Δα measured using DCT are always overlapped by extrinsic components, showing particular long-range variations. The extrinsic components should be eliminated from the measured Δd/d and Δα for quantitative characterization of silicon single crystals. A sample-rotation and area-detector-traverse (RT) method, applicable to X-ray optics for DCT, has been newly developed. The extrinsic components are eliminated by modification of the intensity distribution on the X-ray topographs using the RT method. From theoretical considerations, it is confirmed that the extrinsic components are mainly due to the (+,−) non-parallel setting between the monochromator and the collimator, and a minute bend in the sample due to its physical restraint.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0021889899015484
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  • 7
    Electronic Resource
    Electronic Resource
    Demonstration of X-ray linear dichroism imaging with hard X-rays (2000)
    Chester : International Union of Crystallography (IUCr)
    Journal of synchrotron radiation 7 (2000), S. 368-373 
    Details
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: X-ray polarization-contrast images resulting from X-ray linear dichroism (XLD) in the hard X-ray region have been successfully recorded for the first time. The apparatus used consisted of an X-ray polarizer, double X-ray phase retarders and a high-spatial-resolution X-ray charge-coupled device (CCD) detector. The sample used was a hexagonal close packed (h.c.p.) cobalt single-crystal foil of thickness about 12 µm. The experiment was performed at X-ray energies of 23 and 29 eV above the cobalt K edge (7709 eV), at which the maximum linear dichroisms (∼3%) were observed, with their signs reversed, in the XLD spectrum measured with quadruple X-ray phase retarders. The contrasts in the images at the two X-ray energies were reversed as a result of the XLD in the sample. Furthermore, the values of the contrast in the images arising from the linear dichroism (∼3%) were in good agreement with those yielded by the XLD spectrum.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0909049500009985
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  • 8
    Electronic Resource
    Electronic Resource
    Double electron transitions [MN] or [MO] above 74W MIII edge in X-ray absorption spectra (2001)
    Chester : International Union of Crystallography (IUCr)
    Journal of synchrotron radiation 8 (2001), S. 249-251 
    Details
    ISSN: 1600-5775
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: The X-ray absorption spectra in metallic W (tungsten) have been measured above MIII edge using the third generation synchrotron radiation, ALS, Berkeley. Several features caused by multi-electron transitions were detected on MIII in solid phase. [3p4f], [3p5s], and [3p5p] double electron transitions were identified by the Z+1 approximation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0909049501000760
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  • 9
    Electronic Resource
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    Direct-method-aided phasing of MAD data (2001)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 57 (2001), S. 250-253 
    Details
    ISSN: 1399-0047
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: The direct methods of breaking the phase ambiguity intrinsic in one-wavelength anomalous scattering (OAS) data and MAD phasing are powerful methods in their own rights. In a different context, in addition to their success in phasing OAS data, direct methods can also be useful in the treatment of MAD data. The idea has been tested with the MAD data at 2.5 Å resolution from the protein human adenosine kinase [Mathews et al. (1998), Biochemistry, 37, 15607–15620]. The results showed that the incorporation of direct methods in MAD phasing led to a significant improvement of phases over those obtained from the conventional MAD phasing method alone, as indicated by improved map correlation coefficients (with the existing model), reduced phase errors by 4.5° and improved map connectivity.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0907444900018606
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  • 10
    Electronic Resource
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    Solving crystal structures from two-wavelength X-ray powder diffraction data – breaking the phase ambiguity in the noncentrosymmetric case (2000)
    [S.l.] : International Union of Crystallography (IUCr)
    Acta crystallographica 56 (2000), S. 592-595 
    Details
    ISSN: 1600-5724
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Direct methods of breaking phase ambiguities in protein crystallography have been introduced in powder diffraction analysis. This is aiming at ab initio solution of noncentrosymmetric structures using two-wavelength anomalous powder diffraction data. The known structure of the hydrogen bromide salt of leotidine (C14H20O2N2·HBr) in space group P212121 was used for simulating two-wavelength anomalous powder diffraction with the Br atom as anomalous scatterer. X-ray wavelengths are selected at \lambda_1 = 0.920 and \lambda_2 = 1.500 Å. Unique reflections from the diffraction pattern of \lambda_2 were able to locate the Br atom accurately. All overlapping diffraction peaks were uniformly partitioned to decompose into single reflections. Structure-factor amplitudes were then extracted. With these and the substructure of Br atoms, unique phases for centric reflections (hk0, h0l and 0kl) and phase doublets for noncentric reflections were obtained. The direct method was used to break the phase ambiguity leading to an interpretable electron-density map, from which five cycles of Fourier iteration yielded the complete structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0108767300011417
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