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.
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
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
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).
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
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
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
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