http://cci.lbl.gov/~rwgk/Dissertation/

Dissertation ETH No. 11422

Zeolite Structure Determination from Powder Data:

Computer-based Incorporation of Crystal Chemical Information

The dissertation is available as:

Abstract

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 be developed. Rietveld refinement techniques can be used to extract structural details from a powder diffraction pattern, provided an approximate structure is known. However, if a structural model is not available, its determination from powder diffraction data is a non-trivial matter. The structural information contained in the diffracted intensities is obscured by systematic or accidental overlap of reflections in the powder pattern. As a consequence, the application of structure determination techniques which are very successful for single crystal data (primarily direct methods) is, in general, limited to simpler structures. Since zeolite structures tend to be too complex for this approach, the determination of their structures is still very much dominated by model building. An idea frequently suggested has been to include the active use of the same information on which model building is founded in an automated procedure. The realization of this idea was the predominant goal of this research project.

The FOCUS method, which incorporates the use of some of the chemical information used in model building 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 demonstrated with six test examples of medium to high complexity (zeolite topologies DOH, LEV, RSN, AFR, LTA, EMT). The proportion of overlapping reflections in these examples ranged from 15% to 83%, but the correct topology could be recovered in all cases. Furthermore, the examples show that the most frequently occurring topology produced by FOCUS is, in general, the correct solution, and that the procedure for extracting the intensities from the powder pattern plays a vital role in the outcome of a solution attempt.

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. Preliminary Rietveld refinements of the VPI-9 and VPI-10 structures indicate that the proposed models are correct. The structure of VPI-9 has since been confirmed with a full Rietveld refinement, and the Structure Commission of the International Zeolite Association has assigned the code VNI to that topology. Refinements for VPI-10 and B2 are in progress.

Experience gathered during the course of this project 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. However, from the outset it has been foreseen that the basic idea of using crystal chemical information should also be applicable to other classes of materials. Two examples which show the consequences of relaxing the structural assumptions are presented. 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.

The crown jewel of my Doktorhut:
The automatic FOCUS environment for writing a PhD thesis
Ralf W. Grosse-Kunstleve <rwgk@cci.lbl.gov>