New approaches for solving crystal structures from powder diffraction data

The determination of crystal structures from single crystal diffraction data can generally be carried out routinely and straightforwardly. However, many crystalline solids can be obtained only as microcrystalline powders and are not suitable for investigation by conventional single crystal diffraction methods. In the past, this problem has limited the ability to elucidate the structural properties of such materials. For the wide range of materials in this category, there is clearly a pressing need to develop and exploit techniques that allow crystal structures to be solved from powder diffraction data. Although traditional techniques for structure solution from powder diffraction data have been applied successfully in several cases, these techniques have certain intrinsic limitations, and for the case of organic molecular crystals the challenges that must be overcome are particularly severe. For these reasons, our recent research has focused on the development and implementation of new methodologies for structure solution from powder diffraction data, leading to new "direct-space" techniques for structure solution in which a hypersurface based on the profile R-factor is searched using Monte Carlo or Genetic Algorithm techniques. This paper presents a brief overview of the problems and challenges associated with structure solution from powder diffraction data. The foundations of the techniques that we have developed are described, and illustrative examples (from the field of organic molecular crystals) are given to highlight the application of these techniques.