[Ifeffit] a basic (?) EXAFS question

Matt Newville newville at cars.uchicago.edu
Sun Apr 17 14:52:18 CDT 2005


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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