Ambiguous structure determination from powder data: four different structural models of 4,11-di-fluoro-quinacridone with similar X-ray powder patterns, fit to the PDF, SSNMR and DFT-D.

Four different structural models, which all fit the same X-ray powder pattern, were obtained in the structure determination of 4,11-di-fluoro-quinacridone (CHNOF) from unindexed X-ray powder data by a global fit. The models differ in their lattice parameters, space groups, , ', molecular packing and hydrogen bond patterns. The molecules form a criss-cross pattern in models A and B, a layer structure built from chains in model C and a criss-cross arrangement of dimers in model D.

Electron diffraction, X-ray powder diffraction and pair-distribution-function analyses to determine the crystal structures of Pigment Yellow 213, C23H21N5O9.

The crystal structure of the nanocrystalline alpha phase of Pigment Yellow 213 (P.Y. 213) was solved by a combination of single-crystal electron diffraction and X-ray powder diffraction, despite the poor crystallinity of the material.

Rietveld refinement of a wrong crystal structure.

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.