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@manchester.ac.uk) School of Chemical Engineering and Analytical Science (CEAS) & School of Chemistry The University of Manchester PO Box 88 Sackville Street Manchester M60 1QD United Kingdom http://www.slmslab.info Tel +44 (161) 306 4502 Lab +44 (161) 306 4486 Fax +44 (161) 306 4399 Offices: School of CEAS: Room C17 (Jackson's Mill) School of Chemistry: Room E3 (Faraday Undergraduate Block) DISCLAIMER The views expressed within this message are those of the sender, not those of The University of Manchester or one of its Departments. While all emails and attachments are scanned for viruses before sending, we cannot accept any responsibility for viruses, so please scan all emails and attachments. This email is intended for the addressee only. If you are not the intended recipient, please notify the sender and delete this email immediately. END OF MESSAGE
-----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: 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@ill.fr
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