The crystal structures of two industrially produced yellow organic azo pigments were determined from X-ray powder diffraction data.

Powder patterns were recorded on a STOE-STADI-P-diffractometer equipped with a curved Ge(111) monochromator using CuK_a1-radiation in transmission mode.Powder patterns were indexed using the program DICVOL91 [1] with triclinic unit cells. The structure solution from powder data was difficult because from the crystal symmetries it was obvious that the asymmetric unit contained one organic anion and one Ca cation, with the anion in general position; but it was not known if the Ca ions are situated on general or special positions (inversion centre). Furthermore the number of water molecules per unit cell was not known.

These difficulties were overcome by using combination of direct space methods and Rietveld refinement for structure solution. The crystal structures were solved in an iterative approach using a combination of simulated annealing in DASH [2] to determine the positions of the molecular ions followed by multiple partial Rietveld refinements in TOPAS [3] to locate the Ca ions and missing water molecules. This combination was made easier by the recently developed link between the programs DASH and TOPAS, which automatically generates an input file for TOPAS including all restraints. Finally the structures were Rietveld refined with the use of restraints (bond lengths, bond angles and planarity of aromatic ring systems) with TOPAS.

          1:  R= CH₃                  n = 1 - 3

          2:  R= Cl

[1] A. Boultif & D. Louër, J. Appl. Crystallogr. 24 (1991), 987.

[2] W.I.F. David, K. Shankland, J. van de Streek, E. Pidcock, W. D.S. Motherwell & J.C. Cole, J. Appl. Cryst. 39 (2006), 910.

[3] A.A. Coelho, TOPAS-Academic 4.0 Version 4 (2007).

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