[Ifeffit] a basic (?) EXAFS question

[Juraj Majzlan]

Matt,

thanks a lot for the few simple questions. One of them -
the R range - seems to be the problem, since the range
wasn't big enough. Oh well ... The structure of loellingite
has very well defined shells, up to the fifth one.

Thanks again,

Juro

On Sun, 17 Apr 2005 14:52:18 -0500 (CDT)
 Matt Newville <newville at cars.uchicago.edu> wrote:
> 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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----------------------------------
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