Powder diffraction techniques are becoming increasingly popular as tools for the determination of crystal structures. The authors of this paper have developed a software package, named PowderSolve, to solve crystal structures from experimental powder diffraction patterns and have applied this package to solve the crystal structures of organic compounds with up to 18 variable degrees of freedom (de®ned in terms of the positions, orientations, and internal torsions of the molecular fragments in the asymmetric unit). The package employs a combination of simulated annealing and rigid-body Rietveld re®nement techniques to maximize the agreement between calculated and experimental powder diffraction patterns. The agreement is measured by a full-pro®le comparison (using the R factor R wp ). As an additional check at the end of the structure solution process, accurate force-®eld energies may be used to con®rm the stability of the proposed structure solutions. To generate the calculated powder diffraction pattern, lattice parameters, peak shape parameters and background parameters must be determined accurately before proceeding with the structure solution calculations. For this purpose, a novel variant of the Pawley algorithm is proposed, which avoids the instabilities of the original Pawley method. The successful application and performance of PowderSolve for crystal structure solution of 14 organic compounds of differing complexity are discussed.

Crystal structure determination frequently is a prerequisite for the rational understanding of the solid state properties of new materials. Even though single crystal diffractometry is the method of choice when it comes to crystal structure determination, this approach is often impractical because of the difficulties involved in growing single crystals of appropriate size. High quality powder samples, on the other hand, are much easier to obtain. Using direct-space structure solution techniques, increasingly complex crystal structures can nowadays be solved directly from powder diffraction data. Combined with easy-to-use tools for model building and visualization as well as molecular mechanics and first principles Density Functional Theory (DFT) calculations, crystal structure solution from powder diffraction data is becoming a routine task. To illustrate the applicability of direct-space Monte Carlo techniques to the crystal structure solution of organic and inorganic compounds, a variety of structure solutions with the Powder Solve algorithm are presented. Recent advances include the determination of a preferred orientation correction during the structure solution search and the use of parallel tempering, a newly implemented global search algorithm. As a complementary technique, first principles DFT calculations have been used successfully to validate structure solutions and to aid the subsequent Rietveld refinement.

No abstract

conference abstracts s444 ECM Nancy Acta Cryst. (2000). A56 (Supplement), s444 s10.m1.p19.la Powder Solve-a complete package for structure determination from powder diffraction data.