September 2016 - Ifeffit - millenia.cars.aps.anl.gov

Help in background removal with Athena
by Irina Pi 06 Sep '16

06 Sep '16

Dear all, I am sorry to bother you with my tiny problems, but I have been struggling with it for a long time. The thing is I have XAS data from a graphite sample, and I don't know if I'm plotting the background correctly in it with Athena software, which determines a lot the response in the Fourier transform, and thus in our results. I'm attaching the data as data_exafs.txt, and also two different Athena projects of different backgrount fittings, graphite_exafs1.prj and graphite_exafs2.prj. Just in case projects cannot be seen, I'm also attaching screenshots (1-01deg.png and 2-01deg.png) of the different background fittings and the parameters used in Athena in only one set of data (01deg), because they are the same in the others (02deg and 35deg). Also, the final R-space plot in both fittings are sent (1-rspace.pdf and 2-rspace.pdf). What I wanted to ask is why it is so different the result according to the background removal. And are they correct the fittings I've done? I would like to have a background line similar to the other figure attached (0deg.pdf, but take into account that it is from different data), but I cannot get it. I guess this last fitting seems better, doesn't it? What could I do to have good results in the Fourier transform? Again, sorry but I don't have much experience and it seems to be so subjective that I fear doing it all wrong. Thank you very much, Best regards, Irina Pi Jauma.

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Re: [Ifeffit] need help to get better fitting (Dali Yang)
by Dali Yang 06 Sep '16

06 Sep '16

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 <yunxulanl(a)gmail.com> > To: ifeffit(a)millenia.cars.aps.anl.gov > Subject: [Ifeffit] need help to get better fitting > Message-ID: > <CAKGyrcG_CuCNQDBWQc1b48XQks33VJgdnMnF20RhEo8MHo1GPw(a)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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need help to get better fitting
by Yun Xu 05 Sep '16

05 Sep '16

*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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Co fitting questions
by Neil M Schweitzer 02 Sep '16

02 Sep '16

I have been trying to fit a Co(OH)2 reference sample and think I have a good fit, but I have a couple reservations about it and wanted to run it by some people for an expert opinion. If you are interested, I have included the Athena file, an image of the fit, and the fit log file. Please read on! If not, sorry I spammed you. As you can see, the fit contains four paths. They are all single scattering paths, and the paths with the four largest ranks according to feff. All paths share the same SO2 and deltaE. Each path has its own delR and ss. After some work, I got it to the point that the fit looks pretty (R-factor<0.02), but I have several reservations about it: 1) Generally, the ss values are all higher than I would like. 0.007 even seems high to me for a metal-O bond. Am I right about this? Would this indicate that this may have been a poor choice for reference sample (I should mention it is a powder) and it is not very crystalline? Could it mean there was something we overlooked when we recorded the data? 2) The biggest problem with the fit is the large error on ss_O2 and ss_O3. The largest feature at ~2.75 is obviously Co-Co scattering, but the Co-O paths help to fit the shoulder (or feature at ~3.1 in the Real part) in R-space. If I take them out, then the Co path has a large ss with an equally large error. So here is my question. If I add more paths, the error of ss_O2 and ss_O3 will go down, but then the errors on the new path will consequently be large, so when do I stop adding paths? For the purpose of my samples, I am only interested in the first Co-O path and the Co-Co path. Is this fit good enough you would publish it? Please feel free to critique any other aspect of the fit too. Thanks for your input! Neil

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Larch user base?
by Sean Fackler 01 Sep '16

01 Sep '16

Hello all! Do you have an idea of Larch user base or Larch package download stats? Thanks for any information. Regards, Sean Fackler, PhD Joint Center for Artificial Photosynthesis Lawrence Berkeley National Lab 1 Cyclotron Rd Mail Stop 30R0205 Berkeley CA, 94720 Mobile: 609-613-8734

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