Solving complex zeolite structures from powder diffraction data

To understand the technologically important properties of zeolites and their analogs, structure analysis is essential. However, most new zeolites are prepared in polycrystalline form, so powder diffraction methods of structure solution must be applied, and these have their limitations. In an effort to extend the range of structural complexity that can be addressed when only powder diffraction data are available, a number of new approaches have been explored in recent years. Three that were developed in our research group are described. The first involves the active use of chemical information in an automated structure solution process (focus); the second, the generation of structure envelopes to facilitate structure solution in direct (model-building) space; and the third, the exploitation of preferred orientation (texture) to obtain better estimates of the relative intensities of overlapping reflections. The focus approach is specific to zeolites, but the other two are generally applicable. All three methods have been applied to real problems, and examples are given. The determination of the structure of the high-silica zeolite UTD-1F, with 117 atoms in the asymmetric unit, using the texture approach demonstrates the power of powder diffraction techniques in structure analysis.