International
Tables for
Crystallography
Volume H
Powder diffraction
Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk

International Tables for Crystallography (2018). Vol. H, ch. 3.5, p. 286

Section 3.5.4.2. Structure solution

A. Le Baila*

aUniversité du Maine, Institut des Molécules et Matériaux du Mans, UMR CNRS 6283, Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France
Correspondence e-mail: lebail@univ-lemans.fr

3.5.4.2. Structure solution

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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 |Fhkl| 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[link] is reconsidered. As stated by Pawley, from plausible extinctions the space group of the C6F10 crystal structure at 4.2 K could well be P21/n. The |Fhkl| 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 P21/n of the data rebuilt in P2/m is satisfactory (Fig. 3.5.1[link]). The three-dimensional C6F10 molecule was rotated and translated (six degrees of freedom) in the cell using the ESPOIR (Le Bail, 2001[link]) Monte Carlo program, leading to a plausible starting model (Rp = 13.6%) ready for Rietveld refinement. This program builds a pseudo powder pattern from the extracted |Fhkl| values, which is then compared to the data calculated from the model (Fig. 3.5.2[link]). Unrefined atomic coordinates are available from the Crystallography Open Database (COD, CIF No. 3500009) (Grazulis et al., 2009[link]); a projection of the corresponding structure is shown in Fig. 3.5.3[link]. The true crystal structure is apparently more complex (Solovyov et al., 2014[link]). 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[link]).

[Figure 3.5.1]

Figure 3.5.1 | top | pdf |

Data reduction to |Fhkl| values for the C6F10 Pawley (1981[link]) test case by the Le Bail method using FULLPROF. The neutron powder pattern (4.2 K) was rebuilt (λ = 1.909 Å) from the intensities given in the original paper (P2/m). The extraction of |Fhkl| values was carried out in the space group P21/n.

[Figure 3.5.2]

Figure 3.5.2 | top | pdf |

The C6F10 Monte Carlo molecule positioning by the real-space ESPOIR program produces that best fit (Rp = 13.6%) of the pseudo powder pattern built from the previously extracted |Fhkl| values (Fig. 3.5.1[link]), overcoming the equipartition problem at the reduction stage. Compared to Fig. 3.5.1[link], which shows intensities, the multiplicity and geometrical factors are removed, leading to structure-factor amplitudes.

[Figure 3.5.3]

Figure 3.5.3 | top | pdf |

Projection along the b axis of the C6F10 structure model in P21/n before Rietveld refinement.

References

Grazulis, S., Chateigner, D., Downs, R. T., Yokochi, A. F. T., Quirós, M., Lutterotti, L., Manakova, E., Butkus, J., Moeck, P. & Le Bail, A. (2009). Crystallography Open Database - an open-access collection of crystal structures. J. Appl. Cryst. 42, 726–729.Google Scholar
Le Bail, A. (2001). ESPOIR: A program for solving structures by Monte Carlo analysis of powder diffraction data. Mater. Sci. Forum, 378, 65–70.Google Scholar
Pawley, G. S. (1981). Unit-cell refinement from powder diffraction scans. J. Appl. Cryst. 14, 357–361.Google Scholar
Smrčok, Ľ., Mach, P. & Le Bail, A. (2013). Decafluorocyclohex-1-ene at 4.2 K – crystal structure and theoretical analysis of weak interactions. Acta Cryst. B69, 395–404.Google Scholar
Solovyov, L. A., Fedorov, A. S. & Kuzubov, A. A. (2014). Complete crystal structure of decafluorocyclohex-1-ene at 4.2 K from original neutron diffraction data. Acta Cryst. B70, 395–397.Google Scholar








































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