[Ifeffit] Stoichiometry from EXAFS data

[Scott Calvin]

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