[Ifeffit] a basic (?) EXAFS question

[Kelly, Shelly D.]

Hi Juro,
 
Another thing to try, is to simply sum up all the paths from the crystal structure.  Then compare the chi(k), and real and mag of the FT.  Then plot the individual contriubtions from each of the paths to see how many are significant and how the different paths are contibuting to the overall model.  If the model with all the paths is way different from the data then there may be a problem with the structure.
 
SK

________________________________

From: ifeffit-bounces at millenia.cars.aps.anl.gov on behalf of Matt Newville
Sent: Sun 4/17/2005 2:52 PM
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] a basic (?) EXAFS question



Hi Juro,

I think you'd have to give us more information about how you did
the fits. A few obvious questions are:
   what R range did you use for the fit?
   what was floated and what was fixed in the fit?
   how far out does the data go in k-space?

I'm afraid that I don't know the crystallographic details of
loellingite, but the EXAFS for many crystal structures get very
complex by the fourth shell.  This is especially true in
slightly disordered structures where multiple-scattering and
single paths nearly overlap.  Part of the reason for this is
that multiple-scattering paths can "spread out" in R-space,
making it difficult to know where to stop including paths.

--Matt

On Sun, 17 Apr 2005, Juraj Majzlan wrote:

> Hello,
>
> I have a question about fitting an EXAFS dataset. The
> compound is FeAs2, crystalline, and will be used as a model
> for further study. I was able to fit the first three shells
> very well. However, when I try to fit the fourth shell, I
> either get amplitude that is too high (> 2, with large
> errors), or 0.0. There are several single-scattering and
> several multiple-scattering paths in that region.
> Sometimes, only excluding one of the MS paths leads
> immediately from amplitude of >2 to 0.0.
>
> I'd appreciate any advise about what is the meaning of
> these changes and the inability to fix the spectrum.
>
> Attached are few pictures from Artemis.
>
> Thank you,
>
> Juro Majzlan
>
>
> ----------------------------------
> Juraj Majzlan
> Institute of Mineralogy and Petrology
> Albert-Ludwig-University of Freiburg
> Albertstrasse 23b
> Freiburg, D-79104, Germany
> telephone +49-761-203-6416
> fax +49-761-203-6407
> http://www.minpet.uni-freiburg.de
>


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