[Ifeffit] Coordination of ball-milled alloys

[Alejandro Fernandez-Martinez]

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