Papers by Keyword: Structure Solution

Abstract: EXPO2009 [1], the updated version of EXPO2004 [2], performs all the steps for the ab-initio crystal structure solution by powder diffraction data in a completely automatic way: unit cell determination, space group identification, decomposition of the experimental pattern into single integrated intensities, structure solution by traditional (Direct/Patterson) or direct space/hybrid approaches, structure refinement by Rietveld techniques [3]. New procedures have been introduced in EXPO2009 to strengthen its phasing ability, especially for organic compounds and for low resolution data, for which traditional approaches may fail.

Abstract: The TOPAS macro language can be a powerful tool for increasing the capabilities of X-ray powder diffraction analysis. New corrections and constraints can be implemented without altering the program's code, allowing for experimentation with new ideas and approaches. Examples are given, exposing the power and flexibility of the macro language to help solving problems with a few lines of code. The use of simulated annealing for structure solution of an organic material from data exhibiting preferential orientation is one example. Another one is about extraction of useful structural information in Rietveld refinement of natural hydrotalcite-group minerals, a problematic case that would normally be regarded as over-parameterized for the data available.

Abstract: Solids that can be prepared only as microcrystalline powders are not suitable for structural investigation using single crystal diffraction techniques, and it is necessary instead to carry out structure determination using powder diffraction data. In this paper, we focus on a direct-space strategy for solving crystal structures directly from powder diffraction data in which a hypersurface based on the powder profile R-factor R_wp is searched using a Genetic Algorithm, and we highlight some recent fundamental developments relating to this methodology.