title: Rietveld refinement of a wrong crystal structure.
abstract: Rietveld refinements are generally used to confirm crystal structures solved from powder diffraction data. If the Rietveld refinement converges with low R values and with a smooth difference curve, and the structure looks chemically sensible, the resulting structure is generally considered to be close to the correct crystal structure. Here we present a counter example: The Rietveld refinement of the X-ray powder pattern of gamma-quinacridone with the crystal structure of beta-quinacridone gives quite a smooth difference curve; the resulting crystal structure looks reasonable in terms of molecular conformation, molecular packing and intermolecular hydrogen bonds. However, neither the lattice parameters, the molecular packing nor the conformation of the molecules show any similarity with the actual structure, which was determined from single-crystal data. This example shows that a successful Rietveld refinement is not always final proof of the correctness of a crystal structure; in special cases the resulting crystal structure may still be wrong.
authors.affiliation: Institute of Inorganic and Analytical Chemistry, Johann Wolfgang Goethe University, Max-von-Laue-Strasse 7, D-60438 Frankfurt am Main, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Buchsbaum
authors.forename: Martin U
authors.initials: MU
authors.lastname: Schmidt
mesh_terms: D018360:Crystallography, X-Ray / Q000379:methods*; D006576:Heterocyclic Compounds, 4 or More Rings / Q000737:chemistry; D008958:Models, Molecular; D015394:Molecular Structure; D033363:Neutron Diffraction; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
chemical_list: D006576:Heterocyclic Compounds, 4 or More Rings; C484590:quinacridone