Stoichiometry from EXAFS data
Dear Feff users, lately, we measured a sample containing Sb and Te at EXAFS beamline CEMO, Hasylab and are wondering whether one can determine the stoichiometry from the height of the different K edges´ steps, if one normalizes the values on the edge steps of the elements (cf. http://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html). The absorption gases and the specimen were not changed for the different K edges. Of course, I have already tried doing so and from statistical reproducibility and from the resulting values compared to the expected ones I would estimate an error of this method of about 1%. A source of error that I could imagine originates from the different beam position at different energies combined with a slight inhomogenity in the pressed sample powder. Now my question is: Are there any other sources of error that I should take into accout? Is there any reference on this method from a more experienced user that I could cite? Kind regards, Peter --- Dipl.-Phys. Peter Zalden I. Physikalisches Institut IA, RWTH Aachen, 52056 Aachen EMAIL: zalden@physik.rwth-aachen.de
Hi Peter, I've done this as well, and compared to reliable methods (e.g. ICP). I'd be skeptical of 1%. It's generally quite difficult to determine edge steps to that accuracy. Assuming you're using Athena to determine the edge step, find the most extreme pre- and post-edge lines that seem acceptable and note the range of edge steps. That will yield an uncertainty range. If you have strong features at the white line and just past it, I'd be surprised if you can do much better than 10%. If features in that region are small, such as you might have in an intermetallic alloy, then you might get down to the sub-5% range. While I think that determining the edge step is likely the major source of error, you also have to be aware of the usual suspects in XANES analysis, such as the presence of harmonics in transmission or self-absorption in fluorescence. Testing for linearity with tricks like putting sheets of aluminum foil before I0 can help detect some (but not all) of those kinds of issues. --Scott Calvin Sarah Lawrence College On Aug 2, 2010, at 4:03 AM, Peter Zalden wrote:
Dear Feff users,
lately, we measured a sample containing Sb and Te at EXAFS beamline CEMO, Hasylab and are wondering whether one can determine the stoichiometry from the height of the different K edges´ steps, if one normalizes the values on the edge steps of the elements (cf. http://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html) . The absorption gases and the specimen were not changed for the different K edges. Of course, I have already tried doing so and from statistical reproducibility and from the resulting values compared to the expected ones I would estimate an error of this method of about 1%. A source of error that I could imagine originates from the different beam position at different energies combined with a slight inhomogenity in the pressed sample powder. Now my question is: Are there any other sources of error that I should take into accout? Is there any reference on this method from a more experienced user that I could cite?
Kind regards, Peter
Hi Scott, thanks a lot for your quick response! I found your suggestion very helpful and tried to change the edge step to both extremes by tuning the fitting range for the pre- and post-edge lines in Athena. Due to the very flat structure in the XANES range (cf. attachement), I could modify the value for the edge step by 3% total, which corresponds to an error of +/-2%. One could possibly discuss if this value represents the one-sigma or maybe the two-sigma interval, but the error is nicely small anyway. In the last campaign, we measured a sample of Sb_2Te_1 in two different annealing conditions and from those different data sets (as concerns the EXAFS range), I determined the stoichiometries: Sb_2.06Te_0.94 and exactly the same for the second sample. Therefore, a sub-5% error seems reasonable to assume for these semi-metallic systems. Concerning the influence of higher harmonics: The beam was usually detuned to 70% intensity of the main reflection so that this should not have a strong influence, since the amount of detuning was not changed for both edges. Best regards, Peter Am 02.08.2010 12:52, schrieb Scott Calvin:
Hi Peter,
I've done this as well, and compared to reliable methods (e.g. ICP). I'd be skeptical of 1%. It's generally quite difficult to determine edge steps to that accuracy. Assuming you're using Athena to determine the edge step, find the most extreme pre- and post-edge lines that seem acceptable and note the range of edge steps. That will yield an uncertainty range.
If you have strong features at the white line and just past it, I'd be surprised if you can do much better than 10%. If features in that region are small, such as you might have in an intermetallic alloy, then you might get down to the sub-5% range.
While I think that determining the edge step is likely the major source of error, you also have to be aware of the usual suspects in XANES analysis, such as the presence of harmonics in transmission or self-absorption in fluorescence. Testing for linearity with tricks like putting sheets of aluminum foil before I0 can help detect some (but not all) of those kinds of issues.
--Scott Calvin Sarah Lawrence College
On Aug 2, 2010, at 4:03 AM, Peter Zalden wrote:
Dear Feff users,
lately, we measured a sample containing Sb and Te at EXAFS beamline CEMO, Hasylab and are wondering whether one can determine the stoichiometry from the height of the different K edges´ steps, if one normalizes the values on the edge steps of the elements (cf. http://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html). The absorption gases and the specimen were not changed for the different K edges. Of course, I have already tried doing so and from statistical reproducibility and from the resulting values compared to the expected ones I would estimate an error of this method of about 1%. A source of error that I could imagine originates from the different beam position at different energies combined with a slight inhomogenity in the pressed sample powder. Now my question is: Are there any other sources of error that I should take into accout? Is there any reference on this method from a more experienced user that I could cite?
Kind regards, Peter
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Hi again, Your tellurium edge in particular has the kind of nice regular structure around the edge that makes edge step determination relatively accurate. Also, you're working at such high energies that harmonics are unlikely to be much of an issue. I'd probably consider your data as good to +/- 3% for stoichiometry. Calling that "sigma" or "two sigma" kind of implies that the error in treating many similar systems in this way would be normally distributed, and that's very unlikely to be true. Unlike with population statistics or counting statistics, you wouldn't occasionally end up way off "by chance." It's almost more like a report of precision: when carefully measuring a length with a ruler marked in mm, it's reasonable to interpolate between the mm marks and report a measurement as good to, perhaps +/- 0.3 mm. If, by eye, I claim 11.3 mm, it might conceivably be 11.0 or 11.6 mm, in part because of the ability to eyeball it, and in part because of problems with lining up marks using rulers. But unlike with Gaussian statistics, where two- or three-sigma events happen now and then, I'd often be off by 0.2 mm yet never off by 0.6 mm. --Scott Calvin Sarah Lawrence College On Aug 2, 2010, at 8:51 AM, Peter Zalden wrote:
Hi Scott,
thanks a lot for your quick response! I found your suggestion very helpful and tried to change the edge step to both extremes by tuning the fitting range for the pre- and post-edge lines in Athena. Due to the very flat structure in the XANES range (cf. attachement), I could modify the value for the edge step by 3% total, which corresponds to an error of +/-2%. One could possibly discuss if this value represents the one-sigma or maybe the two-sigma interval, but the error is nicely small anyway. In the last campaign, we measured a sample of Sb_2Te_1 in two different annealing conditions and from those different data sets (as concerns the EXAFS range), I determined the stoichiometries: Sb_2.06Te_0.94 and exactly the same for the second sample. Therefore, a sub-5% error seems reasonable to assume for these semi- metallic systems. Concerning the influence of higher harmonics: The beam was usually detuned to 70% intensity of the main reflection so that this should not have a strong influence, since the amount of detuning was not changed for both edges.
Best regards, Peter
Hi Peter, I would agree Scott's comments. I have used the edge step and fluorescence amplitude to measure both absolute and relative sample compositions. The 10% number he gives seems like a reasonable estimate of the accuracy without comprehensively characterizing the sample and the beamline. For Sb and Te, I could be convinced that the accuracy may be better 5% since some of the sources of error are minimized at high energy; that is, harmonics are likely to be small at 60 keV and 90 keV. I am assuming an edge step between perhaps 0.2 and 2.0. Smaller and larger edge steps will likely have larger uncertainties. A subtle effect that will have a larger effect of the absolute composition than the relative composition is sample uniformity on all length scales. Typically one would only measure uniformity with millimeter beam size. A sample may look relatively uniform on this length scale. However, non-uniformity (for example, pinholes) on a smaller length scale could have a noticeable impact on a measurement of the absolute composition, and possibly on relative measurements, although I have worked this out. Since your edge energies are very close and the high energy means thick samples, the non-uniformity effects will be minimized, assuming you have made samples with sufficiently small particle size. In all, due to the high energy edges, the proximity of the edges, and the magnitude of the edge step, I think your measurement probably is accurate to better than 5%. To do as well as +/-2% determined purely from a study of normalization, I think you have to make certain assumptions about the beamline (harmonics, beam profile, beam motion) and the sample preparation (particle size, microscopic uniformity), if you haven't made the measurements. Jeremy Kropf Argonne National Laboratory
-----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit-bounces@millenia.cars.aps.anl.gov] On Behalf Of Peter Zalden Sent: Monday, August 02, 2010 3:04 AM To: ifeffit@millenia.cars.aps.anl.gov Subject: [Ifeffit] Stoichiometry from EXAFS data
Dear Feff users,
lately, we measured a sample containing Sb and Te at EXAFS beamline CEMO, Hasylab and are wondering whether one can determine the stoichiometry from the height of the different K edges´ steps, if one normalizes the values on the edge steps of the elements (cf. http://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html). The absorption gases and the specimen were not changed for the different K edges. Of course, I have already tried doing so and from statistical reproducibility and from the resulting values compared to the expected ones I would estimate an error of this method of about 1%. A source of error that I could imagine originates from the different beam position at different energies combined with a slight inhomogenity in the pressed sample powder. Now my question is: Are there any other sources of error that I should take into accout? Is there any reference on this method from a more experienced user that I could cite?
Kind regards, Peter --- Dipl.-Phys. Peter Zalden I. Physikalisches Institut IA, RWTH Aachen, 52056 Aachen EMAIL: zalden@physik.rwth-aachen.de
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participants (3)
-
Kropf, Arthur Jeremy -
Peter Zalden -
Scott Calvin