Solving complex zeolite structures from powder diffraction data

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