# [Ifeffit] ODP: Ifeffit Digest, Vol 88, Issue 12

Zajac, Dariusz A. dariusz.zajac at desy.de
Tue Jul 27 10:47:36 CDT 2010

```Dear Rana,
sorry for really late response, but I was cut off from the internet for a while...
if I understand correctly you created a model of the structure and you fit it to the first shell (oxygen?), what is the expected difference between different path? with my humble experience you can try to clarify following problems:
- is your model really correct and cover all possible scenarios?
- are you sure that in your sample are no other (unwanted) phases? with Mn, Fe it can happen...
- have you tried to increase the number of variables? (to free the fit from boundaries)

I am only thinking about it (with an examples at Fe site):
you have 4 positions for Fe and if there is a mixing between Fe and Mn sites additional 3, it means maximally 7 sets of (e0, delr, and ss2 - N you can probably estimate, calculate). that means you have more variables than independent points. you should try, in the first approximation, to free the delr parameter and keep others the same. I havent seen good example of fitting such samples, but if you start from different points, and during fitting you will minimize no. of variables with the same result (with the same fit) should be ok.
good luck
darek

Od: ifeffit-bounces at millenia.cars.aps.anl.gov w imieniu Jatinkumar Rana
Wysłano: Śr 2010-06-16 12:57
Do: ifeffit at millenia.cars.aps.anl.gov
Temat: Re: [Ifeffit] Ifeffit Digest, Vol 88, Issue 12

Dear Darek,

Sorry for the late response. To fit the "first shell" the No.of
independent points and No.of variables which i use are as follows:

I use deltaK=11, deltaR = 1.1 so No.of independent points are 8 and
No.of variables are 4 (i.e. N, e0, delr, and ss2) I set s02=0.9.

Best regards,
Rana

On 14.06.2010 19:00, ifeffit-request at millenia.cars.aps.anl.gov wrote:
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> than "Re: Contents of Ifeffit digest..."
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> Today's Topics:
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>     1. XAFS fitting with multiple atomic sites of absorbing atom
>        (Jatinkumar Rana)
>     2. ODP: XAFS fitting with multiple atomic sites of absorbing
>        atom (Zajac, Dariusz A.)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Mon, 14 Jun 2010 12:00:27 +0200
> From: Jatinkumar Rana<jatinkumar.rana at helmholtz-berlin.de>
> To: Ifeffit users<ifeffit at millenia.cars.aps.anl.gov>
> Subject: [Ifeffit] XAFS fitting with multiple atomic sites of
> 	absorbing atom
> Message-ID:<4C15FDBB.1050203 at helmholtz-berlin.de>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Dear users,
>
> I have measured XAFS spectra on Mn and Fe edge on my material. The
> material contains 3 different atomic positions for Mn and 4 different
> atomic positions for Fe.
>
> Initially, I tried fitting Fe-edge/Mn-edge spectra using indiviual Fe/Mn
> atomic position one-by-one (i.e. first take one Fe/Mn position,
> calculate Feff paths, consider certain paths to fit specific R-range,
> then take another Fe/Mn position and do the same) but the fit was not
> good visually and in terms of goodness of fit parameters.
>
> Then, i tried fitting Fe-edge/Mn-edge spectra, taking into account all
> Fe/Mn atomic position together i.e. I separately calulated Feff paths
> for each Fe/Mn atomic position and summed up related paths from each
> Feff calculation for fitting a specific R-range. (The assumption being,
> the experimental Fe-edge/Mn-edge XAFS spectra has varying contribution
> from all Fe/Mn atomic position in crystal structure).
>
> Anybody in the group could comment about my approach to fit such problem ?
>
> Is there any specific strategy to fit such problem ?
>
> I would be thankful if you could suggest some literature available on
> such problems.
>
>
> Best regards,
> Rana
>
>
> ------------------------------
>
> Message: 2
> Date: Mon, 14 Jun 2010 13:34:18 +0200
> From: "Zajac, Dariusz A."<dariusz.zajac at desy.de>
> To: "XAFS Analysis using Ifeffit"<ifeffit at millenia.cars.aps.anl.gov>
> Subject: [Ifeffit] ODP: XAFS fitting with multiple atomic sites of
> 	absorbing	atom
> Message-ID:
> Content-Type: text/plain;	charset="iso-8859-2"
>
>
> Dear Rana,
> could you specify how many independent data points and variables you have and you use for fitting?
> thnx
> darek
>
> Od: ifeffit-bounces at millenia.cars.aps.anl.gov w imieniu Jatinkumar Rana
> Wys?ano: Pn 2010-06-14 12:00
> Do: Ifeffit users
> Temat: [Ifeffit] XAFS fitting with multiple atomic sites of absorbing atom
>
> Dear users,
>
> I have measured XAFS spectra on Mn and Fe edge on my material. The
> material contains 3 different atomic positions for Mn and 4 different
> atomic positions for Fe.
>
> Initially, I tried fitting Fe-edge/Mn-edge spectra using indiviual Fe/Mn
> atomic position one-by-one (i.e. first take one Fe/Mn position,
> calculate Feff paths, consider certain paths to fit specific R-range,
> then take another Fe/Mn position and do the same) but the fit was not
> good visually and in terms of goodness of fit parameters.
>
> Then, i tried fitting Fe-edge/Mn-edge spectra, taking into account all
> Fe/Mn atomic position together i.e. I separately calulated Feff paths
> for each Fe/Mn atomic position and summed up related paths from each
> Feff calculation for fitting a specific R-range. (The assumption being,
> the experimental Fe-edge/Mn-edge XAFS spectra has varying contribution
> from all Fe/Mn atomic position in crystal structure).
>
> Anybody in the group could comment about my approach to fit such problem ?
>
> Is there any specific strategy to fit such problem ?
>
> I would be thankful if you could suggest some literature available on
> such problems.
>
>
> Best regards,
> Rana
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> End of Ifeffit Digest, Vol 88, Issue 12
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>
>
>

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