April 2009 - Ifeffit - millenia.cars.aps.anl.gov

Fitting procedure
by abhijeet gaur 18 Apr '09

18 Apr '09

Hi all, Thanks to all the concerned persons for having such a wonderful discussion over fitting procedure. I am able to understand the things better by going through these discussions. But what I am trying to say is that I had first applied the fitting procedure in which I make the parameters of all paths( included in the fit) to vary simultaneously, but the problem is that I am not able to get a good fit and the parameters for different paths are also coming absurd like very low amplitude etc. For days I have been trying this method of fitting but not able to get good results.I tried different models also like Debye & Einstien model, I used the relation del r= alpha* Reff but it doesn't work. Then I switch to this the method in which we fit the first shell, get the parameters fix them and then go for the next shell and in this way e fix the whole spectra. The results I got are extremely good. Thus I am using this method. Abhijeet Gaur

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Multiple data set fit
by Debora Meira 18 Apr '09

18 Apr '09

Dear all, I'd like to know a reference that has multiple set data fit as a function of temperature in nanoparticles. Is the relation of Debye Waller factor (s2 = ss2 + sd2) valid for the higher shells? In particular, can I use the Einstein model for sd2? Thanks in advance, Débora Débora M Meira Universidade Federal de São Carlos Pós-Graduação em Engenharia Química Laboratório de Catálise Tel: (16) 92461134 / (16) 34125935 _________________________________________________________________ Faça já uma busa e ganhe um wink do Messenger. Está esperando o que? É grátis! http://www.ibud.com.br/

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R quality factor in k space
by Cammelli Sebastiano 17 Apr '09

17 Apr '09

Dear iffefit user, I found on a paper (E.A. Stern et al. /Physica B 208&209 (1995) 117 120) the definition of R quality factor as: R_factor ≡ < ∆chi(R space))> = √[ ∑ |chi_C(Ri) – chi_E(Ri)|2 / ∑( chi_E(Ri))2] ----> formula 3 Where Chi(Ri) is a complex function (imaginary and real part of the XAFS spectra in the R space), C concerns the calculated XAFS spectrum, while E refers to the experimental XAFS spectrum. In the case of a linear combination fitting on the k space performed by ATHENA, the <∆chi> needs a correction. Is it correct to write: R_factor ≡ < ∆chi(k space))> = √[ ∑ (chi_C(ki) – chi_E(ki))2 / ∑( chi_E(ki))2] ? Where Chi_C(ki) = x1*chi1(k)+x2*chi2(k) : chi1 and chi2 are the EXAFS functions of the two reference samples used for the linear combination procedure and x1, x2 (with 0<x1,x2<1 and x1+x2=1) are their weight; Chi_E(k) is the EXAFS experimental function of the investigated sample. Best regards Sebastiano Cammelli

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Debye-Waller factors for metal oxides ?
by Ian James Drake 16 Apr '09

16 Apr '09

Hi, There is little discussion over the significance of fitted Debye-Waller factors (ss2) found for metal centers in metal oxides of heterogeneous catalysts. I am searching for a better way of determining how reasonable these values are in the fits I get. 1. Are phenomenological models like the correlated Debye model and Einstein model appropriate? It seems that Debye temperatures are hard to find for oxides. 2. Is there an equivalent value for ss2 in x-ray crystallography data that I can refer to for comparison in these materials? What should I look for in reviewing published x-ray crystallographic data? 3. Is the temperature dependence reported for the ss2 for some metals (like Cu and Al) similar to their respective oxides? For example, in Debye-Waller factor calculations reported for Al metal by R.C.G Killean (in J.Phys.F: Metal Phys., v. 4, pg. 1908, 1974), the ss2 changes by a factor of three between 25C and 300C. Would I expect a factor of three increase in crystalline zeolites (AlO4 structural units). 4. Specific to my work: I have studied Al containing oxides at temperatures between 25 and 300 C. I would like to quantify coordination changes by doing EXAFS fitting of the Al K-edge data. The EXAFS was taken at temperature and I observe no change in the broadening of the data. Likewise, the fitting shows no sensitivity in the Debye-Waller factor. It is nearly constant at 0.001 A over the temperatures of interest. Since ss2 and the coordination number (CN) are correlated, I would like a way to bind the error on the fitted CN by modeling real physical changes in the Debye-Waller factors. Do you have any suggestions? Looking forward to your thoughts. Ian Drake Graduate Student UC Berkeley

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Fwd: Questions about Athena & FEFF
by Bruce Ravel 15 Apr '09

15 Apr '09

Pascal, I am answering your question on the Ifeffit mailing list, which is the most appropriate venue for questions of this sort. Please read this page: http://cars9.uchicago.edu/iffwiki/BruceRavel/FormLetters/HelpRequest If I understand your question, you are asking about whether it is appropriate to run Feff with a small cluster. The canonical answer to that question is that you want to use a large cluster whenever possible so that the muffin tin potential is well constrained. Sometimes, with a very small cluster, the algorithm Feff uses to compute the muffin tin radii can result in radii that are unphysically large, resulting in an unphysical calculation. Of course, that is a somewhat hand-waving answer. It is hard to know without actually running a calculation whether the problem with muffin tin radii will actually happen. I have comfortably published results which use very small clusters indeed. In your case, as in so many cases, the best advice I can give is to try it both ways and see what happens. That is, you can try doing a XANES calculation (Are you using Feff8? You didn't say...) with the small cluster and comparing it to the Feff8 calculation which uses a large cluster to compute the potentials and small cluster to compute the full multiple scattering XANES. See the Feff document at http://leonardo.phys.washington.edu/feff/wiki/index.php?title=FEFF_Document… for details about how to set parameters in the feff.inp file to make that happen. Regards, B ---------- Forwarded Message ---------- Subject: Questions about Athena & FEFF Date: Wednesday 15 April 2009 From: "Pascal G. YOT" <pascal.yot(a)univ-montp2.fr> To: bravel(a)bnl.gov Dear Prof. Ravel, I'm working on data coming from XAFS measurements carried out at Pb-L3 edge on lead containing glassy samples. Three reducing agent are used to reduce lead(+II) inside the original glass at high temperature. Our samples contain then a fraction is in fcc lead(0) form and one other fraction of lead(+II) still remaining inside the glassy framework. EXAFS analysis show a decrease of the Pb-O coordination number from 3 (for original glass) to lower CN (1 in extreme case). My major concern is the analysis of XANES, some modifications are observed on the finger prints. I've try to fit the spectra using linear combination fitting module from Athena using lead and original glass spectrum with poor result as you have understand. I need then to generated model XANES spectra for the CN. As I'm not an expert of FEFF, I need some advices. Can I use a cluster PbOx ((d=2.23 A) with coordinate for Pb and O coming from lead oxide) containing few atoms? Is the obtained spectra reliable to give a "good" fit? I hope my questions are clear for you. Thanks in advance for your remarks. Best regards Dr. Pascal Yot ------------------------------------------------------- -- Bruce Ravel ------------------------------------ bravel(a)bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/

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Basic questions
by haiyan 15 Apr '09

15 Apr '09

Dr. Kelly, May I get a copy about setting k-range? Thanks1 Haiyan Zhao Chemical Engineering Department Virginia Tech Blacksburg, VA 24060 Hi Andy, I have a book chapter that details how to set the k-range for a data set. It also talks about Rbkg in detail. If you want a copy just send me an email. Cheers, Shelly

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residual saving
by Guvenc Akgul 15 Apr '09

15 Apr '09

dear all, I need save residual as k^3*Chi(k) but I could not save it because it is saving only as Chi(k).how can do for this? thanks in advance guvenc

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High SO2
by abhijeet gaur 12 Apr '09

12 Apr '09

Hello Scott Sir, I got that ifeffit basically works on the principle of making the theoretical data look similar to experimental data. But what I know about fitting is that we have to first fit the first shell and then keeping the first shell parameter constant , we go for the next shell. Is it not right. Please tell me Abhijeet Gaur

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High SO2 value
by abhijeet gaur 09 Apr '09

09 Apr '09

Hi all, I am fitting a copper foil. I fit the first shell by using first ss path and got it fitted nicely. Then keeping the parameters for the first shell to be fixed , I tried to fit the second shell by taking first and second ss but it is always coming a littel displaced and not fitting well also. But when I added the the third ms path to it , the fit becomes very nice. But the problem is that parameters for third path are coming very bad. And one more problem is that the value of SO2 for second shell is coming very high around 1.5. I am not able to understand the results. Thanks in advance Abhijeet Gaur

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Linear combination fitting
by Guvenc Akgul 09 Apr '09

09 Apr '09

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

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