[Ifeffit] Linear combination fitting

[Guvenc Akgul]

dear all,

thanks for your help.

guven

----- Original Message -----
From: Dominik Samuelis <d.samuelis at fkf.mpg.de>
Date: Tuesday, April 7, 2009 3:18 pm
Subject: Re: [Ifeffit] Linear combination fitting
To: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>


> 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".
> 
> Cheers,
> dominik
> 
> 
> 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
> > _______________________________________________
> > Ifeffit mailing list
> > Ifeffit at millenia.cars.aps.anl.gov
> > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
> >
> 
> 
> 
> 
> -- 
> 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
> Germany
> Phone +49-711-689-1769
> Fax   +49-711-689-1722
> Web   http://www.fkf.mpg.de/maier/
> 
> _______________________________________________
> Ifeffit mailing list
> Ifeffit at millenia.cars.aps.anl.gov
> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit