[Ifeffit] Exafs spectra Fitting of WO3 nanoparticles (Abdallah Nassereddine)

Thu Aug 4 02:08:15 CDT 2022

Dear Abdallah,

It is often quite reasonable to fix two paths debye waller factors to be the same if they are the same atom at similar distances. However there is a slight problem with what you are trying to do. Because EXAFS is a sum of sine waves the minimum distance it can resolve between two paths of the same atom type is Pi/2*Kmax where kmax is the maximum k value in your data. Hence if you are trying to fit separate paths of the same atom type 0.06 A apart you need data out to about 26 k.  

I think and no one has ever told me this is completely Ok the only way to get two distances from such an analysis is to assert that the difference between the path is a fixed value and thus refine the two paths with the same delta R (and probably the same debye wller factor) this in my experience works in some cases
Best regards
Fred Mosselmans

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Today's Topics:

   1. Exafs spectra Fitting of WO3 nanoparticles (Abdallah Nassereddine)

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Message: 1
Date: Wed, 03 Aug 2022 14:40:44 +0200
From: Abdallah Nassereddine <abdallah.nassereddine at esrf.fr>
To: ifeffit at millenia.cars.aps.anl.gov
Subject: [Ifeffit] Exafs spectra Fitting of WO3 nanoparticles
Message-ID: <ddfc8c51f227c914d472d4115733b956 at esrf.fr>
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Dear all,

Good morning,

I try to Fit EXAFS spectra of a sample composed of supported tungsten oxide ( WO3/ZSM-5) nanoparticles obtained by operando studies under gas (CH4, He) at 750 ?C.

First, and after several trials, I analyzed the WO3 reference spectrum obtained at room temperature and I succeeded in determining the amplitude (S0) by using the monoclinic structure WO3 cif file.

Then, I used the value of this amplitude obtained to be able to fit the structural parameters (coordination number N, Debye-waller Factor, Interatomic distance) of WO3/ZSM-5 nanoparticles under gas, but I can't. 
  the best result I got was by selecting the first two shells (at 1.858 and 1.919 Angstrom) of the monoclinic structure. However, with all the optimizations, the Debye-waller factor obtained for the second W-O shell
(1.919 Angstrom) is always negative, and if I don't take this shell into account, I completely lose my Fit.

Have any of you ever worked on this type of sample and might have an idea that would help me improve my Fit?

Thank you in advance,

Best regards,

Abdallah Nassereddine
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