[Ifeffit] Stoichiometry from EXAFS data

[Peter Zalden]

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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