POWDER DIFFRACTION DATA AND CRYSTAL-CHEMICAL INFORMATION COMBINED IN AN AUTOMATED STRUCTURE DETERMINATION PROCEDURE FOR ZEOLITES

The FOCUS method, in which both crystal chemical information and powde r diffraction data are included in the structure determination process , is presented. FOCUS combines automatic Fourier recycling with a spec ialized topology search specific to zeolites, which can be described a s having three-dimensional four-connected framework structures. The ca pabilities of FOCUS have been tested with seven examples of medium to high complexity. The method was then applied to three novel zeolite st ructures and a promising model could be obtained in each case. Experie nce shows that the approach of using chemical and geometric 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 disadvan tage: any method based on assumptions of certain structural properties is also limited to materials which conform to these assumptions. Exam ples which show the consequences of relaxing the structural assumption s are also given.