The Tricks of the Chameleon. Unexpected Symmetry of the Diffraction Pattern

Abstract : The first step in the structure solution process is the determination of candidate space groups on the basis of the symmetry of the diffraction pattern, in particular through the analysis of the reflection conditions. Sometimes it may happen that the diffraction pattern is indexed in an alternative setting of the space group, in which the reflection conditions appear different. The discrepancy between the observed reflection conditions and those published in the standard setting of space groups may lead the investigator to erroneously assume his sample is less symmetric than it actually is. After discussing a case from the literature where tetragonal samples were incorrectly described as orthorhombic due to an unusual indexing of the diffraction pattern, we present an example in which diffractions from a pyrite sample were indexed in the Pb3 setting of the space group, instead of the standard Pa3 setting, where the large set of strong violations of reflection conditions with respect to those of the standard setting could have been interpreted as a signature of symmetry lowering to P213. The structure being well-known, the real origin of the apparent violation *
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Massimo Nespolo, Makoto Tokuda, Akira Yoshiasa. The Tricks of the Chameleon. Unexpected Symmetry of the Diffraction Pattern. Crystal Research and Technology, Wiley-VCH Verlag, 2019, 54, pp.1900063. ⟨10.1002/crat.201900063⟩. ⟨hal-02270513⟩

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