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[sdpd] Calculation of Fhkl or/and Ihkl by different software
> I just tried to compare four different software:
FindIt, Diamond,
> PowderCell and FullProf to calculate structure for
alpha-alumina using
> ICSD #51687 file (Materials Science Forum(2001),
378, 288-293,Unit
> Cell 4.7597(1) 4.7597(1) 12.9935(3)). I used the
same geometry,
> wavelength, profile parameters, no background, same
space group
> settings. But all four patterns have different
intensity, and structure
> factors also (for software that gives a possibility
to get one). I can
> not understand why the structural factors differ.
The visualized
> structures are exactly the same. Maybe somebody
knows?
Dear Irina,
I do not know how big the differences are, so maybe
some of the differences are not big enough to be
relevant. However, trigonal space groups like R-3c are
more difficult to get right than the more
straightforward lower symmetry space groups. This is
because the matrices representing the symmetry
operators are no longer diagonal, which means that it
becomes important to multiply them in the correct
order (from the left or from the right). Not all
programs get this right, which could explain the
differences you are observing.
Secondly, in trigonal space groups special positions
can contain values like 1/3 and 2/3, which can no
longer be represented accurately in text files but are
truncated to values like 0.3333 and 0.6667, which can
give problems with rounding errors; however, the
atomic coordinates of corundum do not contain these
values, so this cannot be the problem.
And Vincent already mentioned another problem, namely
the confusion over the hexagonal versus the
rhombohedral setting.
If the differences that you observe are substantial
(i.e. big enough that at least one of them must
clearly be wrong), then it is important to first
establish what the correct answer is. I found an
experimental powder pattern of corundum (CuKa) on the
web:
http://www.ccp14.ac.uk/tutorial/powderx/runthr.htm
Using the coordinates from ICSD #73724, which can be
found here:
http://www.crystal.unito.it/mssc2006_cd/tutorials/geometry/geom_tut.html
it can easily be tested which program gives the
correct answer.
I verified that the visualiser Mercury, freely
available for a variety of platforms from
http://www.ccdc.cam.ac.uk/products/csd_system/mercury/
gives the correct simulated powder pattern. The next
version of Mercury, Mercury 1.5, due to be released in
a couple of weeks, will also able to write out
structure factors, and it will also have the option to
include CuKa2.
Kind regards,
--
Dr Jacco van de Streek
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