[Ifeffit] Linear combination fitting

Dominik Samuelis d.samuelis at fkf.mpg.de
Tue Apr 7 15:17:21 CDT 2009

Dear Guvenc,

Shelly and Scott have already replied to this issue, and I fully agree  
with their points. Nevertheless, I'd like to add some points, that  
generally apply to linear component fits.

The huge discrepancy between chi(k) and norm(E) fit very likely is an  
indication that your data is not sufficiently described by your  
standards. It might be that you're either using a standard component  
that is not present in your system, or that you have not only two, but  
three or more components in your system.

There is a number of ways to check which one could be the case. Very  
often it helps to fit the system without the restriction that the  
weights of the standards must sum to one. That way, you effectively  
allow for the determination of a residual component during the fit. If  
your system is sufficiently described by the standards, the weights of  
the standards should sum to a constant value close to unity. The  
difference to unity basically gives the residual.

If you have a non-negligible (or even concentration-dependent)  
residual, PCA might help you to determine how many standard components  
actually are required to fit the system. Be aware, though, that the  
interpretation of PCA results is somewhat complicated. If PCA shows  
that you need 4 linearly independent quantities to fully describe your  
system, it doesn't necessarily mean that your system has 4 components.  
Even temperature differences between scans might manifest as an  
additional "component".


Quoting Guvenc Akgul <gakgul at aps.anl.gov>:

> Dear all,
> I have done Ti doped ZnO EXAFS data analysis but I guess it  is  
> getting mixture structure at high concentration for example 55% Ti  
> doped ZnO.Also I done linear combination fit , but at  fitting space  
> norm(E),ZnO weight is 0.779 and ZnTiO3 is 0.221 then I changed  
> fitting space is chi(k)  ZnO weight is getting 25.2 and ZnTiO3 is  
> getting 74.8 and it is not same like at fitting space norm(E) why?
> regards
> guven
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Dr. Dominik Samuelis
d.samuelis at fkf.mpg.de
Max-Planck-Institut für Festkörperforschung
Max Planck Institute for Solid State Research
Heisenbergstr. 1
70569 Stuttgart
Phone +49-711-689-1769
Fax   +49-711-689-1722
Web   http://www.fkf.mpg.de/maier/

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