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Zeitschrift für Kristallographie - Crystalline Materials

Editor-in-Chief: Pöttgen, Rainer

Ed. by Antipov, Evgeny / Boldyreva, Elena V. / Friese, Karen / Huppertz, Hubert / Jahn, Sandro / Tiekink, E. R. T.


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2196-7105
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Volume 228, Issue 12

Issues

“Forbidden” reflections in neutron diffraction on bismuth metal oxides: symmetry reduction, λ/2 effect or Umweganregung?

Johannes Birkenstock / Gwilherm Nénert
  • Institut Laue Langevin, Diffraction Group, BP 156, 6, rue Jules Horowitz, 38042 Grenoble Cedex 9, France
  • Other articles by this author:
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/ Thorsten M. Gesing
  • Universität Bremen, Chemische Kristallographie fester Stoffe, Leobener Str./NW2, 28359 Bremen, Germany
  • Other articles by this author:
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/ Manfred Burianek / Manfred Mühlberg / Reinhard X. Fischer

Abstract

Large crystals of Bi2M4O9 (M = Fe, Ga) were examined by single-crystal neutron diffraction at the Institut Laue Langevin (ILL) in Grenoble. The crystal structures of these bismuth metal oxides are commonly described as mullite-type compounds in space group Pbam. However, up to several hundreds of reflections were observed in both compounds with significant intensity violating the reflection conditions in this space group. Psi-scans with a short wavelength (0.837 Å at D9) were quasi-continuous and thus provided ambiguous results, hampering the identification of the source of these intensities. With a much longer wavelength (2.364 Å at D10) the Psi scans displayed strong variations of the intensities with small changes of the scan angle Psi. These Psi scan intensities could be well matched by an appropriate calculation with the program UMWEG (Rossmanith, 2003) and thus multiple diffraction turned out to be the source of these extra reflections. As a result the space group Pbam proved to be the correct one. Due to widespread misconceptions the geometrical background of Umweganregung is discussed to considerable detail.

Keywords: Mullite-type BiMO phase; Neutron single crystal diffraction; Violation of reflection conditions; Umweganregung; Multiple diffraction; Ewald’s construction

    About the article

    Published in Print: 2013-12-01


    Citation Information: Zeitschrift für Kristallographie - Crystalline Materials, Volume 228, Issue 12, Pages 611–619, ISSN (Online) 2196-7105, ISSN (Print) 2194-4946, DOI: https://doi.org/10.1524/zkri.2013.1649.

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