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Rietveld refinement of a wrong crystal structure.

Christian Buchsbaum1, Martin U Schmidt

  • 1Institute of Inorganic and Analytical Chemistry, Johann Wolfgang Goethe University, Max-von-Laue-Strasse 7, D-60438 Frankfurt am Main, Germany.

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Summary

Rietveld refinement can incorrectly confirm crystal structures. A successful refinement does not guarantee accuracy, as shown with gamma-quinacridone, highlighting potential errors in powder diffraction analysis.

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Area of Science:

  • Crystallography
  • Materials Science
  • Solid-state Chemistry

Background:

  • Rietveld refinement is a standard method for analyzing powder diffraction data to determine crystal structures.
  • A successful Rietveld refinement, indicated by low R values and a smooth difference curve, typically suggests structural accuracy.
  • Chemically sensible structures derived from Rietveld refinement are generally accepted as correct.

Purpose of the Study:

  • To present a case where Rietveld refinement yielded a plausible but incorrect crystal structure.
  • To demonstrate that a successful Rietveld refinement is not definitive proof of structural correctness.
  • To highlight the limitations of powder diffraction data in complex structural analysis.

Main Methods:

  • X-ray powder diffraction data acquisition for gamma-quinacridone.
  • Rietveld refinement performed using the known crystal structure of beta-quinacridone.
  • Comparison of the refined structure with the actual crystal structure determined by single-crystal X-ray diffraction.

Main Results:

  • The Rietveld refinement of gamma-quinacridone converged with a smooth difference curve and chemically reasonable parameters.
  • The refined crystal structure showed no similarity in lattice parameters, molecular packing, or molecular conformation to the actual structure.
  • The actual crystal structure was independently determined using single-crystal X-ray diffraction.

Conclusions:

  • A successful Rietveld refinement does not always equate to the correct crystal structure.
  • Powder diffraction data, even with successful refinement, can lead to erroneous structural models in specific cases.
  • Verification of crystal structures determined from powder data using alternative methods is crucial.