[Ifeffit] Coordination of ball-milled alloys

[Anatoly Frenkel]

Have you measured Fe edge in this alloy, and if yes, have you tried to fit
the data measured from both Fe and Cr edges simultaneously? I would do it
first, without varying the mean free path. It will definitely reduce
correlations between N and ss2, which seems to be a problem here. Whether or
not it works, your method of imposing a lower bound for the EXAFS DWF using
a twice the XRD DWF is a great idea.

What is also missing in your story is the knowledge of the alloy structure
and homogeneity - are these random, substitutional alloys at this
concentration? A strongly distorted hcp (non closed packed) metal will have
a splite of 6 and 6 neighbors in the first shell - and you may not get the
coordination number right if you model it as a single shell of 12 neighbors.
Just a food for thought.

Anatoly


-----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: Friday, May 04, 2007 2:18 PM
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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