Zefsa II

Data requirements

PXD, Cell parameters and Space Group Symmetry

Once the zeolite is successfully synthesized, efforts should be made to obtain a calcined sample on which to do powder diffraction experiments.  The presence of water or template molecules makes the structure determination with ZEFSA more challenging.  On the other hand, the Si/Al ratio does not seem to influence the success of the method.
The diffraction experiment should be of high quality, such as that which can be obtained with synchrotron radiation.  The pattern has to be indexed to determine the space group symmetry and the the cell parameters with reasonable accuracy (say 0.1 Angstrom).  Since the method is quite fast, in the case that the indexing does not unequivocally determine the symmetry, one can try different cases and possibly single out the correct one.

Which reflections to use?

Not all of the data from the PXD should be used for the structure solution. ZEFSA only uses the T-atoms in the model unit cell, and thus high angle reflections cannot be calculated accurately.  The reflections for interplanar spacing less than 1-2 Angstrom should be eliminated. Generally, one should include 50-150 reflections in the PXD, including the very small angle ones.

Composite reflections

Often the resolution of the experiment is not sufficient to resolve closely spaced reflections.  ZEFSA II is a real space method, and one does not need to separate the overlapping peaks.  In a very simple way, reflections that are known to be too close to be resolved are considered as a composite single peak, whose intensity is the sum of the intensities of the overlapping peaks.

Density: T-atom number

ZEFSA II uses the symmetry specified by the user to generate all T-atom positions within the cell.  Thus, only the asymmetric unit cell is represented.  Of course, one could use a P1 triclinic unit cell, but that would make the solution unnecessarily harder. What is important, and should be determined in the laboratory, is the number of T-atoms in the unit cell. Zeolites usually have 14-17 atoms per 1000 cubic Angstrom of cell volume.  The T-atom density and the space group symmetry usually unequivocally identify the number of T-atoms in the asymmetric unit cell.  ZEFSA II correctly accounts for merging on special positions.  It is possible that more than one combination of unique T-atoms/symmetry/merging give similar densities.  Again, different combinations can be tried independently, eventually to find the correct one.

How important is the PXD?

Attempting structure solution with only the cell parameters and the density very seldom works.  Instead of wasting time trying to sort through hundreds of wrong suggested structures, one should take the (small) time required to prepare the PXD input file and use it in the structure solution.  The preparation of the input file is simple: one can obtain   hkl Mul F^2 data directly from the indexing program (like GSAS).

Normalization convention

ZEFSA II assumes that the F^2 value given in the input data file has already been multiplied by the multiplicity.  ZEFSA II will compare the multiplicity times the calculated intensity at the specified hkl against the value in the input file.  The intensities should be normalized so that the maximum one is 1000.  This ensures that the relative weight between the PXD and the other terms in ZEFSA II is standard.
 

Exotic structures, funny coordination and Faults

ZEFSA II implicitly assumes that all atoms are 4-coordinated and that they bond to four neighbors at the preferential distance of 3.1 Angstrom.  This clearly poses limitations to the applicability of ZEFSA II to exotic compounds like the SB family.  Framework metal substitutions with Rb, Cs, Zn and other metals change the atomic distances and sometimes the coordination numbers.  Also, faulted materials cannot be simply modeled, and therefore are not supported in ZEFSA II.

Extensions of ZEFSA II to such cases are possible, but not going to happen in the forseeable future.  Contributions by interested parties are welcome.  Please visit the Feedback page.


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