International
Tables for Crystallography Volume H Powder diffraction Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk © International Union of Crystallography 2018 |
International Tables for Crystallography (2018). Vol. H, ch. 3.5, p. 286
Section 3.5.4.2. Structure solution^{a}Université du Maine, Institut des Molécules et Matériaux du Mans, UMR CNRS 6283, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France |
SDPD can be undertaken by various approaches, depending on the chemical knowledge of the sample (formula, molecular formula, presence of defined polyhedra…), either directly using the |F_{hkl}| values for structure solution by direct or Patterson methods, or by rebuilding a pseudo powder pattern from them, or by applying fixed profile parameters from the Pawley or Le Bail fits during whole-powder-pattern fitting wherein the structure solution is attempted by real-space methods. In order to illustrate the power of WPPD methods and to show the progress realized over the last 30 years, the decafluorocyclohexene structure that was unsolved in the Pawley method paper of 1981 is reconsidered. As stated by Pawley, from plausible extinctions the space group of the C_{6}F_{10} crystal structure at 4.2 K could well be P2_{1}/n. The |F_{hkl}| values were extracted from the rebuilt neutron powder pattern by applying the Le Bail method and used for attempting the structure solution by real-space methods. The neutron powder pattern was rebuilt from the 109 intensities extracted up to 54° 2θ, in space group P2/m, given in Table 2 of the original paper. The fit (using FULLPROF) in P2_{1}/n of the data rebuilt in P2/m is satisfactory (Fig. 3.5.1). The three-dimensional C_{6}F_{10} molecule was rotated and translated (six degrees of freedom) in the cell using the ESPOIR (Le Bail, 2001) Monte Carlo program, leading to a plausible starting model (R_{p} = 13.6%) ready for Rietveld refinement. This program builds a pseudo powder pattern from the extracted |F_{hkl}| values, which is then compared to the data calculated from the model (Fig. 3.5.2). Unrefined atomic coordinates are available from the Crystallography Open Database (COD, CIF No. 3500009) (Grazulis et al., 2009); a projection of the corresponding structure is shown in Fig. 3.5.3. The true crystal structure is apparently more complex (Solovyov et al., 2014). Final resolution of the structure will require collection of a better experimental powder pattern. However, the coordinates have been refined by energy minimization in the solid state (Smrčok et al., 2013).
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