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@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@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@ill.fr -- Geochimie de l'Environnement LGIT - UJF BP 53 38041 Grenoble Cedex France _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit