[Ifeffit] [IFEFFIT] question about chemically unordered systems

[Carlo Segre]

Dear Falk

You cannot treat this with a crystallography point of view. In this structure, the two sites are basically the same crystallographically and so you will get nowhere by swapping them and generating a new calculation. You must generate a cluster first and then manually swap atoms around or replace them in order to make a disordered environment.

However, in this case you really only have one thing you can measure: that is the ratio of A to B neighbors around A and the ratio of B to A neighbors around B.

In the ideal ordered structure, you will only have A around B and vice versa in the first shell. You can measure this if you have a sufficient contrast between A and B and a theoretical path for each case.

You can do this by generating a FEFF input file for the ordered structure around the target atom, say A and then replacing one or more of the B atoms near it with A atoms before running the FEFF calculation. This will give you two paths with which to work. If you have data for the B atom edge too, you can do the same for its environment and then you can model the two edges together with some constraints. Finally, you can move to the second (and maybe third) shell to get a more extended model. The parameters you obtain can give you a clue as to the degree of disorder.

Carlo
-- 
Sent from my Android phone with K-9 Mail. Please excuse my brevity.

"Meutzner, Falk" <f.meutzner at hzdr.de> wrote:

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 
<newville at cars.uchicago.edu>:
> Hi Falk,
> 
> On Tue, Aug 7, 2012 at 8:42 AM, Meutzner, Falk <f.meutzner at 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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