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abstracts
Rietveld refinements generally fail, if the lattice parameters of the structural model differ more than slightly from the correct lattice parameters and the simulated reflections do not overlap with the experimental ones. For molecular crystals, we have developed a more robust fitting algorithm, which uses the cross-correlation function of calculated and experimental powder patterns, and allows a FIt with DEviating Lattice parameters (FIDEL). The method is also successful for nanocrystalline organic compounds showing only 10-20 peaks in their powder diagrams. The FIDEL method has proven to be useful for various applications, including refinements starting from (1) structure data of an isostructural chemical derivative; (2) structure data of an isostructural hydrate or solvate; (3) structure data from measurements at another temperature (e.g. for fitting a room-temperature powder diagram starting with a structure determined from a single-crystal measurement at 100K). FIDEL is also used for determining crystal structures from non-indexed powder diagrams of nanocrystalline organic compounds. Three steps are performed: (1) Prediction of possible crystal structures in various space groups using global lattice-energy minimizations by force-field methods. (2) FIDEL fit of 100 to 600 low-energy structures to the experimental powder pattern. The structure candidate leading to the correct structure results in a significantly better fit than all other structures. (3) Rietveld refinement. The FIDEL method was used to determine the hitherto unknown crystal structure of the nanocrystalline alpha-phase of 2,9-dichloroquinacridone (C20H10Cl2N2O2). The upper part of the figure shows the experimental powder pattern and the simulated powder diagram of one of the predicted low-energy structures before any fitting. The lower part displays the result of the FIDEL fit, before the Rietveld refinement.
abstracts
Zeitschrift für Kristallographie, Angewandte Chemie, The New York Times, The Sun, El Pais, La Republica, Le Monde, Shanghai Daily, and many more journals and newspapers are printed with Pigment Red 57:1. P.R.57:1 (C18H12CaN2O6S · n H2O, n = 0,1,3) is the most important organic red pigment with a production of more than 50,000 tons per year and an annual sales volume of more than 200 million Euro.[1] In printing ink the pigment is not dissolved, but finely dispersed. Consequently its solid-state properties are maintained. Like most pigments, P.R.57:1 occurs in different crystal phases with different colours. Upon synthesis a trihydrate is formed. Drying at 50 0C generates a monohydrate with magenta shade, which is used for printing inks. The monohydrate is thermally stable up to temperatures higher than 190 0C before it releases water to yield a hygroscopic anhydrous phase with dull dark magenta shade. For all three phases the growth of single crystals is impeded by the low solubility of the pigment in most media. The crystal structures of all three forms were determined from in-house X-ray powder data.[2] The structures were solved by real-space methods with simulated annealing. Subsequently a Rietveld refinement with restraints on bond lengths, bond angles and planar groups was performed. All three phases crystallize in space-group type P21/c, Z = 4. The trihydrate and the monohydrate show eightfold coordination of the Ca ions, the anhydrate a sevenfold one. Apparently the increasing anion-cation interactions lead to the observed colour shift. The arrangement of cations and anions is similar in all three forms. The crystal structures exhibit double layers, one polar, one nonpolar. The polar layer consists of water molecules, calcium ions, sulfonate, keto and carboxylate groups, held together mostly by hydrogen bonds and Coulomb interactions. The nonpolar layer contains naphthalene and toluene moieties.
