[Ifeffit] Re: Amplitude and the EXAFS equation

[Scott Calvin]

Hi all,

I thought I'd add my two cents:

I agree with everything Bruce said (!), but I think it is worth 
emphasizing one aspect of this issue. Basically, unless I'm using a 
mutiple-element fit with some clever constraints (not yet available 
in Artemis, but soon, I gather), any determination of coordination 
number is likely to be pretty uncertain. Not only does N correlate 
completely with S02 for a single shell fit, it is also likely to 
correlate strongly with sigma2 (or the Debye or Einstein temperature 
or whatever I'm using as the free parameter). Multiple-shell fits 
help when I can use them, but if I don't know the coordination number 
for the first shell, how likely is it that I know the structure of 
the material well enough to fit outer shells? Although you may see 
coordination numbers determined by EXAFS with an "uncertainty" of 1 
in publications, this is usually something like the uncertainty 
reported in IFEFFIT--it probabIy doesn't include systematic errors 
like the effect of assumptions concerning the Debye-Waller factor or 
S02. In most cases if I'm looking for a first shell coordination 
number via EXAFS I'm not counting on better than a factor of two 
accuracy. That may still be useful in some cases, but doesn't help 
distinguish between, say, tetrahedral and octahedral coordination.

So, if I'm only looking for a factor of 2, and if I'm pretty sure my 
sample is homogenous and of a consistent and appropriate thickness, 
I'm happy to just constrain S02 to 0.90 or 0.85 or whatever, knowing 
that the arbitrary choice of S02 is probably introducing an 
uncertainty of less than 20% in the coordination number. Since this 
is still considerably less than I generally believe is introduced by 
other issues (such as the scheme used for the Debye-Waller factors), 
it's good enough for me.

--Scott Calvin
Naval Research Lab
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