Many thanks, Anatoly and Matt, for your answers. 1)What happens if you try to fit the whole first peak (bcc 1st and 2nd neighbors) without making assumptions about sigma2? Then I get a coordination number of 5.2 for the first shell (being 14 the theoretical value for a bcc), what sounds to me physically unreallistic. From Rietveld refinement of diffraction data we know that our particles are big enough (75 nm) to have a small surface to bulk ratio and so coordination may not be affected in this dramatic way by boundary effects. This is what induced me to consider constraining the DWF. 2) If you're expecting a bcc structure, you should definitely include the first two shells (~14 neighbors), not trying to resolve the coordination shells with 8 and 6 neighbors separately. Regarding data quality, what I have is a k range up to 10, so certainly not enough resolution to distinguish the first two shells. But if I know that certainly my structure is a bcc (I have good diffraction data), why shouldn't I use the information that I know from my structure by constraining bond distances between certain ranges for 1st and 2nd neighbourghs? After all, they are quite far away (2.50 A vs 2.88 A)... (?). Even coordination of atoms from the 'amorphous matrix' (that is very reduced) should keep distances similar to these... what do you think? 4) About the nature of the alloy, from our experience we know that Cr is behaving in a very similar way to Fe here, so we should have a random substitutional alloy. From high resolution x-ray diffraction data we know that part of the Fe has not been alloyed, so there remain some 'clusters' of Fe. 5) I am not trying to distinguish between between Fe-Cr and Cr-Cr paths, so always using the Cr-Cr paths generated by FEFF. Thanks again for help! Alex Anatoly Frenkel wrote:
...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@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@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@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
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-- 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