[Ifeffit] Coordination of ball-milled alloys

Sven L.M. Schroeder sven.schroeder at gnomikos.com
Wed May 9 04:06:14 CDT 2007


Hi Alex

How did you measure the data? Transmission, fluorescence-yield,
electron-yield?
What was the sample - a foil, a powder, small lumps ...? How did you prepare
the sample for your measurements?

Certain combinations of these factors may introduce spurious XAFS amplitude
distortions/reductions.

What about 'amorphous' oxide overlayers on the grains produced by milling?

Can you analyse your samples with transmission electron microscopy?

Additional food for thought...

Sven

--
Sven L.M. Schroeder (mailto:s.schroeder at manchester.ac.uk)

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> -----Original Message-----
> From: ifeffit-bounces at millenia.cars.aps.anl.gov 
> [mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf 
> Of Alejandro Fernandez-Martinez
> Sent: 04 May 2007 19:18
> To: XAFS Analysis using Ifeffit
> Subject: [Ifeffit] Coordination of ball-milled alloys
> 
> Dear all,
> 
> I am quite new to XAS data analysis, maybe you can help me 
> with a little problem I have.
> 
> I am trying to fit EXAFS data at Cr K-edge of a bcc Fe30Cr70 
> alloy synthesized by ball-milling. For those who don't know 
> it, this technique is very effective in alloying metals but 
> introduces a lot of structural defects in the structure. This 
> is clearly seen in neutron diffraction data (with DW 2 times 
> larger than DWs of a Cr reference and larger FWHMs).
> 
> When fitting the EXAFS data without any specific constrain (only for
> distances) I get DWs on the same order of magnitude for the 
> Cr-foil than for the alloy, and a very reduced coordination 
> of N1=2.3 (compare with
> N1=8 for the Cr-foil) for the first neighbour (without 
> physical sense?).
> 
> In order to take into account the larger DWs for the alloyed 
> sample (as found in diffraction data) I have constrained the 
> minimum value of the DWs to a value double than the DWs of 
> the Cr-foil. This gives me an increase of the N for the first 
> shell, but at the same time, I have to refine lambda 
> (photoelectron mean free path) in order to correct the 
> excesive decay introduced by the DW exponential in the EXAFS 
> formula, and to get a good fit. Then I get a more physically 
> understandable N1=5.5.
> 
> - The 1st question is: do you think the logical discourse 
> that I have followed until now is correct? I don't have idea 
> on how much some quantities as lambda should or not be refined.
> - 2nd question: what dou you think about the use of 
> diffraction data as constraint for EXAFS data analysis?
> 
> Many thanks in advance!
> 
> cheers,
> Alex
> 
> --
> Alejandro Fernandez-Martinez
> Institut Laue-Langevin
> 6, rue Jules Horowitz - B.P. 156
> 38042 Grenoble Cedex 9, France
> 
> Tel: +33 (0)4.76.20.75.71
> Fax: +33 (0)4 76.20.76.48
> fernande at ill.fr
> 
> --
> Geochimie de l'Environnement
> LGIT - UJF
> BP 53
> 38041 Grenoble Cedex
> France
> 
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