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
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
...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 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 _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
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
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
_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
-- 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
Hi Alejandro, A couple of things jump out at me: First, you say the data goes out to 10Ang^-1. That seems short for Cr in a Cr-Fe alloy, and implies either non-ideal data collection (as Sven alluded to: pinholes in a transmission sample, data collected in fluorescence, or a surface-contaminated e-yield measurement), or a highly disordered Cr environment.
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.
To paraphrase, the sample is not a random mixture of Cr and Fe in a bcc lattice, which seems a little strange to me, as bcc Cr and Fe have very similar lattice constants -- I don't know whether they form a uniform solid solution, but you are studying this sample, so it might not be as simple as a random substitution, right??? Do you know the fraction of Fe atoms that has been alloyed? Do you know how much of the sample is crystalline, and what the defect density is? These all might be important: You're assuming that all Cr is surrounded by Cr, in a single bcc environment. Suppose there were two or more Cr environments, with slightly different Cr-metal distances. By assuming 1 environment, you'd end up with a sigma2 that included the static disorder of having multiple environments. .. which could all be related to having such a short data range. By the way, any such issue would completely negate the assertion that there is a simple relation between the diffraction and XAFS sigma^2. The assertion that sigma^2 <= 2 definitely assumes that all atoms are on a single set of lattice points. For example, it is not a very useful guideline for amorphous systems. Hope that helps, --Matt
Thanks for your help. To paraphrase, the sample is not a random mixture of Cr and Fe in a bcc lattice, which seems a little strange to me, as bcc Cr and Fe have very similar lattice constants -- I don't know whether they form a uniform solid solution, but you are studying this sample, so it might not be as simple as a random substitution, right??? Do you know the fraction of Fe atoms that has been alloyed? Do you know how much of the sample is crystalline, and what the defect density is? I know exactly the percentage of Fe atoms that have not been alloyed. They come from the fact that in the ball milling we used powder of Fe with bigger grain size than Cr powder and that the milling time was not very long, so some Fe did not have enough time to alloy. The issue with the short range may bue due to inhomogeneities in the sample combined with the instability of the beam that day (I was one of the first XAS users of that beamline..).
By the way, any such issue would completely negate the assertion that there is a simple relation between the diffraction and XAFS sigma^2. The assertion that sigma^2 <= 2 definitely assumes that all atoms are on a single set of lattice points. For example, it is not a very useful guideline for amorphous systems.
This is a very interesting comment. Thanks a lot. 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
Hi Alejandro, I would not expect lambda to depend on alloy structure -- it's generally expected to be fairly insensitive to structural details. Your assertion that the EXAFS sigma2 should be larger for Cr in a disordered alloy than for Cr in Cr-Cr foil, and coordination numbers should be on the order of 10 for metal alloys is certainly reasonable. Then again, there are a couple of questions and points to consider: 1. 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. 2. For highly disordered systems, diffraction and EXAFS can give quite wildly different results. Most diffraction measurements (as opposed to scattering measurements) implicitly select the crystalline portion of a sample -- amorphous portions may not contribute at all. EXAFS averages equally over all atoms. In some cases, this can be an important distinction. In addition, for liquid metals it does often appear that the sigma2 doesn't change much, but that N drops dramatically. I wouldn't expect that to be a huge effect for Cr-Fe alloys, but it's possible for very high defect concentrations. 3. Related to that, a doubling of a diffraction DWF for statically disorder really does not imply a doubling of the EXAFS sigma2. In fact, the evidence is generally that the EXAFS sigma2 (ie, the range of bond lengths) doesn't really change much, while the increase in diffraction DWF simply implies a loss of long-range (which is, after all, what diffraction DWFs measure). 4. Finally, it's probably impossible to tell Cr-Cr scattering from Cr-Fe. How are you taking into account the ~30% of neighbors that are Fe instead of Cr? So I would suggest that there's probably not a good physical basis for floating lambda, and there really is not compelling argument for forcing sigma2 to be twice that for Cr metal. What happens if you try to fit the whole first peak (bcc 1st and 2nd neighbors) without making assumptions about sigma2? --Matt
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
-- 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
participants (4)
-
Alejandro Fernandez-Martinez
-
Anatoly Frenkel
-
Matt Newville
-
Sven L.M. Schroeder