[Ifeffit] Using the amplitude reduction factor as a linear combination fitting parameter

Wayne Lukens wwlukens at lbl.gov
Tue Apr 29 09:43:30 CDT 2008

Dear Christophe,

The one drawback with using S02 in the way that you describe is that
in so doing, you are assuming the true value of SO2 is 1.  If you have
a good reference spectrum and your reference is a well-defined solid,
you can fit the reference spectrum to its known structure and determine
SO2 for the experiment.  You could then define a parameter, x, in
artemis that is the fraction of UO2 then (1-x) would be the fraction of
uranyl.  The coordination number of each UO2 shell would be multiplied
by x and each uranyl shell by (1-x).

For what it's worth, SO2 for uranium oxides using Feff7 always seems to
be around 0.9 for us if I recall correctly.


Wayne Lukens
Lawrence Berkeley National Laboratory
email: wwlukens at lbl.gov
phone: (510) 486-4305
FAX: (510) 486-5596

Bruggeman Christophe wrote:
> I have recently collected EXAFS spectra of uranium on a FeS2 
> surface. Using principal component analysis of the XANES and k3-weighted 
> EXAFS spectra, I have found that there are two uranium species which 
> compose the spectra. As a first tentative guess, I believe these two 
> uranium species are uraninite (UO2(c)) and a uranyl species. I would 
> like now to fit the fourier transform functions (real parts and 
> magnitudes) using the theoretical paths and path degeneracies created by 
> feff, and use the amplitude reduction factor S02 as a fitting parameter 
> to derive the relative amounts of the two uranium species in my samples.
> Normally, this S02 is taken as a constant (between 0.7 and 1.0), and the 
> path degeneracies are fitted. So normally, S02 is not really a fitting 
> parameter (some papers derive it even with theoretical functions). 
> However, given the fact that S02 and N are completely correlated, I 
> think it is justified to use this approach.
> Can someone comment on this?
> Many thanks in advance,
> Christophe
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