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Orientational disorder of monomethyl-quinacridone investigated by Rietveld refinement, structure refinement to the pair distribution function and lattice-energy minimizations

DOI: 10.1107/S2052520620003984 DOI Help

Authors: Carina Schlesinger (Goethe Universität) , Sonja M. Hammer (Goethe Universität) , Tatiana E. Gorelik (Ulm University) , Martin U. Schmidt (Goethe Universität)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Acta Crystallographica Section B Structural Science, Crystal Engineering And Materials , VOL 76

State: Published (Approved)
Published: June 2020

Abstract: The crystal structure of the organic pigment 2-mono­methyl-quinacridone (Pigment Red 192, C21H14N2O2) was solved from X-ray powder diffraction data. The resulting average structure is described in space group [P\overline 1], Z = 1 with the molecule on the inversion centre. The molecules are arranged in chains. The molecules, which have no inversion symmetry, show orientational head-to-tail disorder. In the average structure, the methyl group is disordered and found on both ends of the molecule with an occupancy of 0.5 each. The disorder and the local structure were investigated using various ordered structural models. All models were analysed by three approaches: Rietveld refinement, structure refinement to the pair distribution function (PDF) and lattice-energy minimization. All refinements converged well. The Rietveld refinement provided the average structure and gave no indication of a long-range ordering. The refinement to the PDF turned out to be very sensitive to small structural details, giving insight into the local structure. The lattice-energy minimizations revealed a significantly preferred local ordering of neighbouring molecules along the [0[\bar 1]1] direction. In conclusion, all methods indicate a statistical orientational disorder with a preferred parallel orientation of molecules in one direction. Additionally, electron diffraction revealed twinning and faint diffuse scattering.

Journal Keywords: disorder; Rietveld refinement; pair distribution function; lattice-energy minimization; electron diffraction

Subject Areas: Materials, Chemistry

Instruments: I15-1-X-ray Pair Distribution Function (XPDF)