[Ifeffit] need help to get better fitting (Dali Yang)

[Dali Yang]

Hi, Mia,

In your Artemis file “Cs1.33Fe1.33_FeKedge_Fit.fpj”, It seems that you are using a Ti atom as the core atom instead of an Fe atom. So, you are trying to fit a Fe_K_edge EXAFS using a "Ti-O” and ”Ti-Ti" path?  That is probably the major problem. However, I tried to replace the core atom in your feff input file, but I can not get a reasonable fitting for the first shell either(see the attachment). I guess you are using a cif file which is not your real Fe dopant sample. The Fe dopant might expand your structure to make Fe-O path(bond length, coordination) very different from Ti-O, so we can not get a decent fitting for this data. Meanwhile, I can not see the meaning by using a Fe foil to extract the S0^2 for your data fitting. 

Good luck,

Dali

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> Message: 1
> Date: Mon, 5 Sep 2016 15:58:47 -0400
> From: Yun Xu <yunxulanl at gmail.com>
> To: ifeffit at millenia.cars.aps.anl.gov
> Subject: [Ifeffit] need help to get better fitting
> Message-ID:
> 	<CAKGyrcG_CuCNQDBWQc1b48XQks33VJgdnMnF20RhEo8MHo1GPw at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
> 
> *Hello, This is Mia From Clemson University.*
> *I have problem with my fitting, this has bothered me for long time.*
> *So the thing is I use Fe metal foil as reference, and I was able to get
> S02 from fitting the Fe metal foil XAS as seen in the attached file
> Fe_metal foil fit.*
> *Then I used this S02 value to fit the Fe_kedge exafs data of Hollandite
> sample, Cs1.33Fe1.33Ti6.67O16. The hollandite crystal structure cif file is
> also attached as hollandite.cif. Fe and Ti sit in the six oxygen
> coordinated octahedral. the crystal structure is shown in the attached
> figure as hollandite.jpg. (neglect the color of octahedra) This cif file is
> the original structure without dopant as BaTi8O16 (I4/m space group). so I
> set the core atom as Fe in the Feff, and assume second and third shell as
> Ti, in the DFT calculation, Fe octahedra tend to have a Ti octahedra as
> neighbor rather than a Fe ocatahedra. *
> *I've also attached my artemis file to see anybody could help me get  a
> better fitting. I just need to fit the range up 3.5A which includes the
> first oxygen shell and Two Ti shell.*
> *A specific question is I always get negative S02 value, can any body tell
> me what I should do if I get negative S02.*
> *Thank you very much*
> *Mia*
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