The mean-square Friedel intensity difference in P1 with a centrosymmetric substructure

H. D. Flack, U. Shmueli

Research output: Contribution to journalArticlepeer-review

Abstract

For non-centrosymmetric structures in space group P1 containing a centrosymmetric substructure, analytical expressions have been obtained for various functions of the diffraction intensity of Friedel opposites. These functions are the average intensity of Friedel opposites, the mean difference in intensity of Friedel opposites and the mean-square difference in intensity of Friedel opposites. A Bijvoet intensity ratio is defined for the evaluation of resonant-scattering effects in non-centrosymmetric and pseudo-centrosymmetric structures. Analysis of these expressions confirms that both resonant and non-resonant atoms are necessary to produce differences in intensity between Friedel opposites and also shows that in some circumstances atoms may lie on a centrosymmetric substructure without diminishing the Bijvoet intensity ratio. The effects of the real component of resonant scattering, of the variation of the scattering factors with sin θ/λ, of isotropic atomic displacement parameters, of a crystal twinned by inversion, of atoms in special positions and of weak reflections are considered. Software is available for the evaluation of the Bijvoet intensity ratio.

Original languageEnglish
Article numbersh5056
Pages (from-to)257-265
Number of pages9
JournalActa Crystallographica Section A: Foundations of Crystallography
Volume63
Issue number3
DOIs
StatePublished - 13 Apr 2007

Keywords

  • Absolute configuration
  • Absolute structure
  • Anomalous scattering
  • Flack parameter
  • Intensity statistics
  • Pseudo-centrosymmetric structures
  • Resonant scattering

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