Dear Matt, dear community
thank you for your fast reply!
I am not sure, if I understand you correctly. I tried what you suggested, but
the result is just the same. I will try to give some more insight:
Our binary system is bcc (pm-3m), there is one atom on the position (0,0,0)
and another one on the position of (0.5,0.5,0.5).
After creation of the material, we have a chemically disordered state. Any of
the two atoms can occupy any of the aforementioned positions.
After heat treatment however, the material chemically orders itself and
becomes CsCl structure. In theory, atom A will always be on the corner at
(0,0,0), while atom B will always be in the centre of the cell at
(0.5,0.5,0.5).
Our problem is, we would like to distinguish between these two phases, because
we possibly have mixtures of the ordered and disordered states.
We already achieved the ordered state with atoms, because they are unique
positions (Atom A at (0,0,0), atom B at (0.5,0.5,0.5). Unfortunately we are
unsure how to model the disordered state. We would need the possibility to
tell the programme, that the observed atom will always be A at (0,0,0), while
the other atom has a possibility of 50% to be A and 50% to be B.
For the programme it seems not to matter which of the positions our observed
atom is at. Therefore the calculated spectra are the same.
I hope I was able to give more insight into our problem.
Thank you! Best Regards
Falk
Am Tue, 7 Aug 2012 09:20:15 -0500 schrieb Matt Newville
<newvi...@cars.uchicago.edu>:
Hi Falk,
On Tue, Aug 7, 2012 at 8:42 AM, Meutzner, Falk <f.meutz...@hzdr.de> wrote:
Dear IFEFFIT Community,
is there a way to calculate the spectrum for chemically disordered systems?
We have a simple binary bcc structure. It crystallises either in
superstructure (CsCl) or chemically disordered (W type) with both atoms
randomly taking part of either (0,0,0) or (0.5,0.5,0.5). It is easy to get
the superstructured model for artemis, but we have no clue how to model the
disordered system...
Thanks a lot for your help, we really appreciate it.
Best regards
Falk
Feff generates the EXAFS contributions for an individual atom in a
cluster of atoms. To model a range of local structures that an atomic
species might have in a disordered (or even moderately complex ordered
structure, such as two inequivalent sites in a unit cell), one has to
do a calculation for each candidate environment for the absorbing
species and average the results accordingly.
To model the disordered system, I think you would need to run one
calculation with the central atom at the (0, 0, 0) site and one at the
(1/2, 1/2, 1/2) site. You can then add the weighted contributions
from the corresponding paths from each calculation. One of the
things EXAFS might be able to distinguish (depending on the Z contrast
of the atoms in your structure) is whether one site is preferred over
another.
I hope that helps get you started. If not, perhaps a more concrete
question would help.
--Matt
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