[Ifeffit] Coordination of ball-milled alloys

[Anatoly Frenkel]

...EXAFS DWF = 2*XRD DWF is actually an upper bound for EXAFS DWF, not the
lower bound, sorry. The difference is the correlation term between the
central atom and nearest neighbor which is negative and probably never
negligible, thus it is an upper bound.

Anatoly


-----Original Message-----
From: ifeffit-bounces at millenia.cars.aps.anl.gov
[mailto:ifeffit-bounces at millenia.cars.aps.anl.gov]On Behalf Of Anatoly
Frenkel
Sent: Friday, May 04, 2007 2:32 PM
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] Coordination of ball-milled alloys


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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