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  • 1
    Electronic Resource
    Electronic Resource
    Copenhagen : International Union of Crystallography (IUCr)
    Applied crystallography online 32 (1999), S. 799-807 
    ISSN: 1600-5767
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Geosciences , Physics
    Notes: In a laboratory X-ray powder diffraction study, the evaluation of the patterns of three Bragg–Brentano powder diffractometers with different monochromator geometries has been undertaken. For the measurements on each diffractometer, the standard reference material SRM 640 (silicon) and the corundum samples SRM 674a and SRM 1976 have been used. In each case, the peak profiles were fitted with a split Pearson VII function and the FWHM (full width at half-maximum) parameters and exponent m were determined for the left (lower 2θ) and the right (higher 2θ) sides of the Bragg peaks. It was found that there is a strong dependence of both the FWHM and the exponent m on the diffraction angle for the two configurations that included monochromators, whereas nearly constant values of m were found for the geometrically simplest diffractometer working without a monochromator. Finally, the two components of the Cu Kα doublet show systematically different peak profiles. There is a clear difference not only concerning the FWHM, which becomes more obvious at higher 2θ values, but also in the course of m with respect to the diffraction angle for the left and the right tails of the powder reflections. This is the main reason for the difficulties in Kα2 stripping and also in single-line-profile analysis when using the Kα doublet. Therefore, it is not surprising that this phenomenon, which can be explained by Heisenberg's uncertainty principle, does affect the reliability (represented by standard R values) of structure refinement by the Rietveld method.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0044-2313
    Keywords: Cesium disodium trihydroxide hexahydrate ; deuterated material ; neutron diffraction ; hydrogen bonding ; crystal structure ; Chemistry ; Inorganic Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Description / Table of Contents: Hydrates of Weak and Strong Bases. IX. Hydrogen Bonding in CsNa2(OD)3 · 6D2O: A Single-Crystal Neutron-Diffraction Study at 120 KThe title compound is the first hydrate of a ternary alkali hydroxide reported. Its structure is related to the hexagonal perovskites and at the same time shaped by a complex system of hydrogen bonds. In a previous X-ray structure analysis at room temperature of the isotypic undeuterated material, these had been characterized only insufficiently. Therefore a complete structure determination has now been performed on a perdeuterated single crystal using low-temperature neutron-diffraction data (Pca21, Z = 4; a = 13.86, b = 6.070, c = 12.41 Å at 120 K; 1 655 observed independent reflections, R = 0.027).
    Notes: Die Titelverbindung ist das erste beschriebene Hydrat eines ternären Alkalihydroxids. Ihre Struktur ist verwandt mit der der hexagonalen Perowskite und zugleich durch ein komplexes System von Wasserstoffbrücken geprägt. Dieses war in einer vorangegangenen Röntgenstrukturanalyse bei Raumtemperatur an der isotypen nicht deuterierten Substanz nur unzureichend charakterisiert worden. Daher wurde nun eine vollständige Strukturbestimmung an einem perdeuterierten Einkristall mit Tieftemperatur-Neutronenbeugungsdaten durchgeführt (Pca21, Z = 4; a = 13,86, b = 6,070, c = 12,41 Å bei 120 K; 1 655 beobachtete unabhängige Reflexe, R = 0,027).
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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