Dear I study a binary alloy (Fe98.7Cu1.3) where a phase change from bcc to fcc is evident (at the Cu K edge of course) and it depends on the annealing time. I have 4 samples A,B,C,D, where the A shows a perfect bcc structure, while the D a clear fcc structure. B and C have structure that is a mix between the fcc and bcc. I found very interesting to perform some linear fit simulations using the A and D of the XANES and EXAFS of the samples B and C. so for instance it is possible to get the xanes of the B sample as linear combinations of A and D according to BXANES = x*AXANES+(1-x)*DXANES and for the EXAFS BEXAFS = y*AEXAFS+(1-y)*DEXAFS The simulations work perfectly but: 1- Of course I expect that x=y (am I wrong?). but this is not true according to the values that the linear fit provide me. There is a small but significant difference as x=48±2% and y=44±2%. Does this difference have any physical meaning? Is the error a bit too small (so should I increase it?) 2- For the sample C, I got an amazing simulation adding some noise (0.001), but when I try to export the data (operation: "Write a report") I cannot get the data corrected with the noise (I can get only 6 columns: k, data, fit, residual, sample D, sample A). Is there any way to get the data from the data+noise curve? Best regards Sebastiano Cammelli
Hi Sebastiano,
I study a binary alloy (Fe98.7Cu1.3) where a phase change from bcc to fcc is evident (at the Cu K edge of course) and it depends on the annealing time. I have 4 samples A,B,C,D, where the A shows a perfect bcc structure, while the D a clear fcc structure. B and C have structure that is a mix between the fcc and bcc. I found very interesting to perform some linear fit simulations using the A and D of the XANES and EXAFS of the samples B and C. so for instance it is possible to get the xanes of the B sample as linear combinations of A and D according to
BXANES = x*AXANES+(1-x)*DXANES and for the EXAFS
BEXAFS = y*AEXAFS+(1-y)*DEXAFS
The simulations work perfectly but:
1- Of course I expect that x=y (am I wrong?). but this is not true according to the values that the linear fit provide me. There is a small but significant difference as x=48±2% and y=44±2%. Does this difference have any physical meaning? Is the error a bit too small (so should I increase it?)
I don't think you are either wrong or right -- it depends upon the details of the system. Were your system a mixture of two powders in the ratios you observe in sample B and C, then you would indeed expect x=y. If I understand your situation correctly, you have an alloy that displays a local structure (FCC-like around the Cu atom) that is different from the bulk structure (presumably BCC since this is mostly Fe). That means that, on some length scale, the correlations in the material change from FCC-like to BCC-like. Suppose that length scale at which the correlations change is 10s of Angstroms. That is too far for the EXAFS-with-an-E to see, but near the edge the mean free path of the photoelectron might be long enough to start seeing that effect. Thus the structure probed by the XANES portion of the spectrum might not, in that sense, be identical to the structure probed by the EXAFS. In that case, you might not expect x=y. The extent of their difference is an indication that the XANES is probing the system on a longer length scale than the EXAFS. I admit that what I just wrote is a bit half-baked -- it's just my initial thought upon reading your email. But it seems plausible.
2- For the sample C, I got an amazing simulation adding some noise (0.001), but when I try to export the data (operation: "Write a report") I cannot get the data corrected with the noise (I can get only 6 columns: k, data, fit, residual, sample D, sample A). Is there any way to get the data from the data+noise curve?
Hmmm... I need to look into the soruce code, but I would guess that I just forgot to apply the noise when writing out the LCF fit results. I doubt that it will be a difficult fix and I will let the list know when I have checked the fix into the SVN repository. It is unlikely that I'll have a chance to make a new Windows executable before August, however. As a work-around, you could generate a noise spectrum yourself and add it to the data outside of Athena. It won't be the same noise spectrum that was used in the fit, but I suspect it'll be hard to tell the difference when you make a plot. As I say in the document "The noise is randomly generated using an epsilon that is chosen as a fraction of the size of the edge step." B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/
I study a binary alloy (Fe98.7Cu1.3) where a phase change from bcc to fcc is evident (at the Cu K edge of course) and it depends on the annealing time. I have 4 samples A,B,C,D, where the A shows a perfect bcc structure, while the D a clear fcc structure. B and C have structure that is a mix between the fcc and bcc. I found very interesting to perform some
fit simulations using the A and D of the XANES and EXAFS of the samples B and C. so for instance it is possible to get the xanes of the B sample as linear combinations of A and D according to
BXANES = x*AXANES+(1-x)*DXANES and for the EXAFS
BEXAFS = y*AEXAFS+(1-y)*DEXAFS
The simulations work perfectly but:
1- Of course I expect that x=y (am I wrong?). but this is not
Sebastiano: Sounds like a martensitic transition since it depends on the annealing time. Thus, as Bruce wrote, XANES and EXAFS may be expected to be different, due to the different lengths scales - if the boundaries of local fcc regions are small. Another origin of this discrepancy in x and y between XANES and EXAFS fits, is Debye-Waller factor. Due to the extra disorder caused by the interface between fcc and bcc region within the sample, EXAFS in the fcc region in B is not equivalent to EXAFS in D. Analogously, EXAFS in the bcc region in B is not equivalent to EXAFS in A. Therefore, linear combination of A and D will not give you B at "correct" mixing fractions (correct, meaning, real physical mixing fraction). To compensate for that, linear combination problem will stil try to get you the betst fit but by expense of changing the x and y from their "correct" values. XANES is giving you a more accurate result in this sense since Debye-Waller factor, even if different between the unknown (B) from A or D, will not affect XANES that much as EXAFS. Anatoly -----Original Message----- From: Ravel, Bruce [mailto:bravel@bnl.gov] Sent: Thursday, June 05, 2008 10:39 AM To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] Linear combination analysis Hi Sebastiano, linear true
according to the values that the linear fit provide me. There is a small but significant difference as x=48±2% and y=44±2%. Does this difference have any physical meaning? Is the error a bit too small (so should I increase it?)
I don't think you are either wrong or right -- it depends upon the details of the system. Were your system a mixture of two powders in the ratios you observe in sample B and C, then you would indeed expect x=y. If I understand your situation correctly, you have an alloy that displays a local structure (FCC-like around the Cu atom) that is different from the bulk structure (presumably BCC since this is mostly Fe). That means that, on some length scale, the correlations in the material change from FCC-like to BCC-like. Suppose that length scale at which the correlations change is 10s of Angstroms. That is too far for the EXAFS-with-an-E to see, but near the edge the mean free path of the photoelectron might be long enough to start seeing that effect. Thus the structure probed by the XANES portion of the spectrum might not, in that sense, be identical to the structure probed by the EXAFS. In that case, you might not expect x=y. The extent of their difference is an indication that the XANES is probing the system on a longer length scale than the EXAFS. I admit that what I just wrote is a bit half-baked -- it's just my initial thought upon reading your email. But it seems plausible.
2- For the sample C, I got an amazing simulation adding some noise (0.001), but when I try to export the data (operation: "Write a report") I cannot get the data corrected with the noise (I can get only 6
columns: k, > data, fit, residual, sample D, sample A). Is there any way to get the data > from the data+noise curve?
Hmmm... I need to look into the soruce code, but I would guess that I just forgot to apply the noise when writing out the LCF fit results. I doubt that it will be a difficult fix and I will let the list know when I have checked the fix into the SVN repository. It is unlikely that I'll have a chance to make a new Windows executable before August, however. As a work-around, you could generate a noise spectrum yourself and add it to the data outside of Athena. It won't be the same noise spectrum that was used in the fit, but I suspect it'll be hard to tell the difference when you make a plot. As I say in the document "The noise is randomly generated using an epsilon that is chosen as a fraction of the size of the edge step." B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 My homepage: HYPERLINK "http://xafs.org/BruceRavel"http://xafs.org/BruceRavel EXAFS software: HYPERLINK "http://cars9.uchicago.edu/~ravel/software/exafs/"http://cars9.uchicago. edu/~ravel/software/exafs/ _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov HYPERLINK "http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit"http://millen ia.cars.aps.anl.gov/mailman/listinfo/ifeffit No virus found in this outgoing message. Checked by AVG. Version: 7.5.524 / Virus Database: 269.24.6/1484 - Release Date: 6/4/2008 4:40 PM
MessageOne possibility is over-absorption, which will affect the XANES and EXAFS a bit differently. mam ----- Original Message ----- From: Anatoly Frenkel To: 'Ravel, Bruce' ; 'XAFS Analysis using Ifeffit' Sent: Thursday, June 05, 2008 7:58 AM Subject: Re: [Ifeffit] Linear combination analysis Sebastiano: Sounds like a martensitic transition since it depends on the annealing time. Thus, as Bruce wrote, XANES and EXAFS may be expected to be different, due to the different lengths scales - if the boundaries of local fcc regions are small. Another origin of this discrepancy in x and y between XANES and EXAFS fits, is Debye-Waller factor. Due to the extra disorder caused by the interface between fcc and bcc region within the sample, EXAFS in the fcc region in B is not equivalent to EXAFS in D. Analogously, EXAFS in the bcc region in B is not equivalent to EXAFS in A. Therefore, linear combination of A and D will not give you B at "correct" mixing fractions (correct, meaning, real physical mixing fraction). To compensate for that, linear combination problem will stil try to get you the betst fit but by expense of changing the x and y from their "correct" values. XANES is giving you a more accurate result in this sense since Debye-Waller factor, even if different between the unknown (B) from A or D, will not affect XANES that much as EXAFS. Anatoly -----Original Message----- From: Ravel, Bruce [mailto:bravel@bnl.gov] Sent: Thursday, June 05, 2008 10:39 AM To: XAFS Analysis using Ifeffit Subject: Re: [Ifeffit] Linear combination analysis Hi Sebastiano, > I study a binary alloy (Fe98.7Cu1.3) where a phase change from bcc to fcc > is evident (at the Cu K edge of course) and it depends on the annealing > time. I have 4 samples A,B,C,D, where the A shows a perfect bcc structure, > while the D a clear fcc structure. B and C have structure that is a mix > between the fcc and bcc. I found very interesting to perform some linear > fit simulations using the A and D of the XANES and EXAFS of the samples B > and C. so for instance it is possible to get the xanes of the B sample as > linear combinations of A and D according to > > BXANES = x*AXANES+(1-x)*DXANES and for the EXAFS > > BEXAFS = y*AEXAFS+(1-y)*DEXAFS > > The simulations work perfectly but: > > 1- Of course I expect that x=y (am I wrong?). but this is not true > according to the values that the linear fit provide me. There is a small > but significant difference as x=48±2% and y=44±2%. Does this difference > have any physical meaning? Is the error a bit too small (so should I > increase it?) I don't think you are either wrong or right -- it depends upon the details of the system. Were your system a mixture of two powders in the ratios you observe in sample B and C, then you would indeed expect x=y. If I understand your situation correctly, you have an alloy that displays a local structure (FCC-like around the Cu atom) that is different from the bulk structure (presumably BCC since this is mostly Fe). That means that, on some length scale, the correlations in the material change from FCC-like to BCC-like. Suppose that length scale at which the correlations change is 10s of Angstroms. That is too far for the EXAFS-with-an-E to see, but near the edge the mean free path of the photoelectron might be long enough to start seeing that effect. Thus the structure probed by the XANES portion of the spectrum might not, in that sense, be identical to the structure probed by the EXAFS. In that case, you might not expect x=y. The extent of their difference is an indication that the XANES is probing the system on a longer length scale than the EXAFS. I admit that what I just wrote is a bit half-baked -- it's just my initial thought upon reading your email. But it seems plausible. > 2- For the sample C, I got an amazing simulation adding some noise > (0.001), but when I try to export the data (operation: "Write a report") I > cannot get the data corrected with the noise (I can get only 6 columns: k, > data, fit, residual, sample D, sample A). Is there any way to get the data > from the data+noise curve? Hmmm... I need to look into the soruce code, but I would guess that I just forgot to apply the noise when writing out the LCF fit results. I doubt that it will be a difficult fix and I will let the list know when I have checked the fix into the SVN repository. It is unlikely that I'll have a chance to make a new Windows executable before August, however. As a work-around, you could generate a noise spectrum yourself and add it to the data outside of Athena. It won't be the same noise spectrum that was used in the fit, but I suspect it'll be hard to tell the difference when you make a plot. As I say in the document "The noise is randomly generated using an epsilon that is chosen as a fraction of the size of the edge step." B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/ _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit No virus found in this outgoing message. Checked by AVG. Version: 7.5.524 / Virus Database: 269.24.6/1484 - Release Date: 6/4/2008 4:40 PM ------------------------------------------------------------------------------ _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
participants (4)
-
Anatoly Frenkel -
Bruce Ravel -
Cammelli Sebastiano -
Matthew