[Ifeffit] Stoichiometry from EXAFS data

[Kropf, Arthur Jeremy]

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 at millenia.cars.aps.anl.gov 
> [mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf 
> Of Peter Zalden
> Sent: Monday, August 02, 2010 3:04 AM
> To: ifeffit at 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 at physik.rwth-aachen.de
> 
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