ZEOLITE CRYSTALLOGRAPHY - STRUCTURE DETERMINATION IN THE ABSENCE OF CONVENTIONAL SINGLE-CRYSTAL DATA

Zeolite molecular sieves are a class of aluminosilicates whose technologically important properties are intimately related to their structures. Since most synthetic zeolites are only available as powders and not as single crystals, conventional crystallographic techniques cannot be used to solve their structures. The crystallographic difficulties are exacerbated by the pseudosymmetry, faulting, disorder and impurity problems, which are relatively common in zeolite materials. Over the years, zeolite crystallographers have developed a number of approaches to these problems and these are discussed. The techniques include informed model building, various computer approaches to model generation, application of direct methods to powder data, microcrystalline diffraction and exploitation of magic angle spinning NMR and electron microscopy. Usually a number of methods are combined to construct a framework structure model and to test it for feasibility. Once a reasonable model has been produced, Rietveld refinement will not only confirm (or reject) the framework proposal but also reveal further details of the structure. Although the techniques described were developed for application to zeolite structures, many can probably be applied to other systems.