Re: [Ifeffit] need help to get better fitting (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

Message: 1 Date: Mon, 5 Sep 2016 15:58:47 -0400 From: Yun Xu To: ifeffit@millenia.cars.aps.anl.gov Subject: [Ifeffit] need help to get better fitting Message-ID: 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*