ZEOLITE STRUCTURE DETERMINATION FROM POWDER DATA: COMPUTER-BASED INCORPORATION OF CHEMICAL INFORMATION. R.W. Grosse-Kunstleve, L.B. McCusker &;Ch. Baerlocher, Laboratory of Crystallography, ETH Zentrum, CH-8092 Zurich.

Many technologically and industrially important materials, including zeolites, are synthesized and used in polycrystalline form. Since the crystal structures of such phases often determine their useful properties, it is essential that methods to study their structures are available.

The FOCUS method, which incorporates the use of chemical information into the structure determination process, has been developed. FOCUS combines automatic Fourier recycling (using integrated intensities extracted from a powder pattern and random starting phases) with a specialized framework search specific to zeolite structures, which can be described as 3-dimensional 4-connected topologies. The capabilities of FOCUS have been tested with six examples of medium to high complexity (zeolite topologies DOH, LEV, RSN, AFR, LTA, EMT), and the method was then applied to three novel zeolite structures - the two zincosilicates VPI-9 and VPI-10, and the beryllosilicate B2 - and a promising model was obtained in all cases. The structure of VPI-9 has since been confirmed with a full Rietveld refinement, and the code VNI has been assigned to that topology. Refinements for VPI-10 and B2 are in progress.

Experience shows that the approach of using chemical and geometrical knowledge can compensate for some of the information that is lost as a result of the overlap problem. At the same time, there is an intrinsic disadvantage: any method based on assumptions of certain structural properties is also limited to materials which conform to these assumptions. Examples which show the consequences of relaxing the structural assumptions were investigated, and it was found that the computing time requirements of FOCUS grow very rapidly with the number of different possible connectivity types. Suggestions for further developments to overcome this problem are outlined.