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

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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New Method for Normalization?
by Barton, David (DG) 30 Mar '09

30 Mar '09

XANES Aficionados, I know most users on this list are hard-core EXAFS analysts, but some of us use XANES analysis to decipher mixed phases or use it to get structural clues when data quality isn't sufficient for EXAFS analysis. Anyway, I have a question for those interested in quantitative analysis of XANES or those that are willing to offer an opinion on methods to normalize raw spectra to get reliable, repeatable comparisons of near-edge features. Has anyone considered using the methods of Penner-Hahn to normalize their data to get reliable intensities of near-edge features? The reference to their method of normalization is below: J. Synchrotron Rad. (2005). 12, 506-510 [doi:10.1107/S0909049504034193] "A method for normalization of X-ray absorption spectra" T.-C. Weng, G. S. Waldo and J. E. Penner-Hahn It is my understanding the Ifeffit uses a linear function to remove the pre-edge and a quadratic for the post-edge and that the Athena's flatten is a subtraction of these functions. This is an excellent method under most scenarios since the background is usually fairly smooth and can be well approximated by a quadratic equation. Personally, I have had only a few rare cases where the quadratic function on the post-edge was not sufficient to reliably normalize the data and in those cases a third-order polynomial was sufficient. Does anyone else have an opinion on using alternative normalization routines? Dave _________________________________ David Barton The Dow Chemical Company

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I got that
by abhijeet gaur 24 Feb '09

24 Feb '09

Thanks Dr. Dominik . I should have tried this on web .Anyways thanks again Sir. Abhijeet

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Attachment
by abhijeet gaur 23 Feb '09

23 Feb '09

Hi Dr. Bruce Actually I got the idea that we had to take into account the multiple scatterring path during fitting. But I am not able open the attachment that you have send , it is some vlc media file. Can you please send it in some other format like pdf.I will be very thankfull to you. Abhijeet

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Neagtive amplitude
by abhijeet gaur 22 Feb '09

22 Feb '09

Hi all, I am fitting a *copper foil* . I am getting a good fit after using the *4 single scattering paths* but in case of *fourth path* I am getting *negative amplitude*. What should be the *probable reason* for that as I am getting a *very good fit*. with thanks Abhijeet Gaur

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Disordered shells
by Eugenio Otal 19 Feb '09

19 Feb '09

Hi all, I need to simulate some structural disorder in the first and second shell of interstitial erbium in ZnO for XANES and EXAFS spectra. Which is the best way to obtain information of the spectra variation with structural changes? If I use DEBYE card, how to correlate temperature with structural parameters? Everything is so quite in the line, no messages since 13/feb... Regards, euG

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Artemis fitting problem
by abhijeet gaur 13 Feb '09

13 Feb '09

Hi every one, Actually I want to know that is it necessary to fit Ei, 3rd, 4th, cumulant in the fitting procedure .If yes then please tell me how to get them involve in the fit .when I give parameter like guess 3rd, guess 4th . Artemis dont take them . One more thing is that I am fitting Cu foil, CuO and Cu2O , so how many number of paths should I take . Abhijeet

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Re: [Ifeffit] SIXPACK
by Matt Newville 13 Feb '09

13 Feb '09

Hi Bounif, I'm actually not responsible for the PCA analysis part of SixPACK, Sam Webb (the author of SixPACK!) is. And I have to admit I don't know all the details beyond what is documented at: http://www-ssrl.slac.stanford.edu/~swebb/spdocs/sixpack_documentation.htm#P… I'll try to answer your questions, and hopefully Sam or someone on this list will correct me: > My question is the following one: > When I calculate the principals components from the data matrix ( sampls ) > I must make a target transformation to obtain the réels compoents which > constitutes mixtures. > 1-a reels components are obtained by the transformationof the principals > components or de target components? I believe it uses the components found from PCA as the pieces with which to build a spectra that matches the target. > 2-How to calculate the concentration of the reels constituents ? I am not sure how to get those numbers directly. If the target transformation works to reproduce your spectra, you could always then try linear combinations to get the sample constituents. I'm not sure that this is guaranteed to work or give a unique answer (say, if all of your samples were truly mixtures of multiple components themselves), but I think that it does normally work in practice with a small number of components and some of them "nearly" endmembers. Cheers, --Matt Newville <newville at cars.uchicago.edu> 2009/2/11 bounif <bounif(a)univ-paris12.fr>: > Dear Newville, > With big enjoyment which I writes you,I am a student in paris 12 > université,I work on the decomposition of the spectre EXAFS of mixtures at > several pure constituents,I test various methods, Among these methods the > principals components analisis(PCA).For this, I use the SIXPACK software > which you are the author. > My question is the following one: > When I calculate the principals components from the data matrix ( sampls ) > I must make a target transformation to obtain the réels compoents which > constitutes mixtures. > 1-a reels components are obtained by the transformationof the principals > components or de target components? > 2-How to calculate the concentration of the reels constituents ? > Very cordially > > > cher Newville, > avec grande joie que je vous écrit,je suis un doctorant à luniversité de > paris 12,je travail sur la décomposition des spectre EXAFS des mélanges en > plusieurs composantes pures,je test différentes méthodes,parmi ces > méthodes l'analyse en composantes principale.pour cela j'utilise le logiciel > SIXPACK dont vous êtes l'auteur. > Ma question est la suivante:quand je calcule les composantes principales > (components) à partir de la matrice des donnés (sampls) je dois effectuer un > transformation pour obtenir les composés réels qui constitue les mélanges. > 1-les composés réel sont obtenus par la transformation des composantes > principales où les composantes cible (target ) ? > 2-comment calculer la concentration des composantes réel? > bien cordialement > > > > > > Bounif Mohamed > > Institut de Chimie et des Matériaux Paris Est (ICMPE) > UMR CNRS-Université Paris 12 n° 7182 > Equipe SAX > 2 à 8 Rue Henri Dunant, 94320 THIAIS France > Tel : +33.1.49.78.13.38 > Fax : +33.1.49.78.12.03 > > > > >

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Re: [Ifeffit] Mo EXAFS
by Matt Newville 13 Feb '09

13 Feb '09

Dear Mehdi, Sorry for taking so long to respond, and hope you'll again forgive me for forwarding this message to the Ifeffit mailing list. I actually have taken very little data on Mo compounds. I don't know what is in the Lytel database for the compounds you list, either. But I'll bet that someone on this list can help you out.... --Matt Mehdi Monir <MXM785(a)bham.ac.uk> wrote: > Dear Matthew > > I am a PhD research student in metallurgy and materials dept. at University of Birmingham . I am working under supervision of Dr. Alison J. Davenport (University of Birmingham) and Prof. Trevor Rayment(Diamond light source). > We are working on a project which is part of an EPSRC -funded program to characterize the chemistry and structure of salt films on Fe, Ni and stainless steel in chloride base solutions with and without other anions, and to explore the use of X-Ray absorption spectroscopy for characterizing chemistry of pits in these materials. > We have done XAS experiments on Mo edge this september at Diamond light source(Synchrotron Facility, Oxfordshire, UK).I am looking for Mo standard spectra like Mo foil, MoAcetate, MoCl3, MoO2, MoCl5 and Na2MoO4 in HCl solutions but unfortunately couldn't find too many ref spectra. > It would be really great help if give me any idea or introduce any resources to find these information. > > Many Thanks > > Mehdi > >

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Co doped znO
by Guvenc Akgul (gakgul) 13 Feb '09

13 Feb '09

dear all I want to figure out and plot theorical Co doped ZnO using FEFF8. how can I do it? thanks guven

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