[Ifeffit] Fe oxide

Scott Calvin scalvin at slc.edu
Mon Sep 26 09:56:40 CDT 2005

Hi Juro,

Off-hand, it sounds like you might be trying to fit a square peg into a
round hole, so to speak. If the precipitate is amorphous, do you have any
reason to strongly suspect a goethite-based structure? 

I must admit, I haven't looked at your Artemis file yet, so I'm not
positive what you mean by "fitting them separate" as opposed to
"simultaneously," but you're probably not going to be able to resolve peaks
that close...one commonly used criteria is that

delta k > pi/[2(r2-r1)]

where r2 and r1 are the locations of the peaks and delta k is the width of
the k-range you're sampling. So the 0.16 A difference between the higher
two points requires

delta k > 10 A-1

which your data probably doesn't quite support. The fact that you get high
uncertainties in the distances when you fit them separately tends to
confirm that.

By the way, note that the inability to resolve peaks doesn't mean you
should lump them together when you know (or strongly suspect) they should
be there...it just means you can't have separate parameters for them. For
instance, two nearby peaks that are treated as one will have an anomalously
large sigma^2 and possibly an anomalous third cumulant as well...if you
have a good enough model, it is better to treat them as separate peaks with
the same number of free parameters (e.g. constrain their sigma^2's to be
the same, and perhaps fix the separation between them or something like
that). But in your case I'm not sure you have strong reason to expect there
really are separate peaks...

--Scott Calvin
Sarah Lawrence College

At 03:46 PM 9/26/2005 +0200, you wrote:

>Can I expect to resolve these three Fe shells or is it
>asking too much?

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