[Ifeffit] Commercial MoS2 -- Correction

[Scott Calvin]

Hi Dan,

A couple of quick thoughts, that might also be helpful to people starting
out with EXAFS fitting.

First of all, the good news is that changing the k-weight does not seem to
affect the values ifeffit finds for the parameters significantly (i.e. I
didn't see any that moved outside of their uncertainties). That's very
reassuring. The fact that the fit quality is noticeably better at k-wt 3
suggests to me that you might try raising kmin a bit--maybe the k-wt 3 fit
works better because the fits are struggling at the low end of the k-range?
Sometimes it's hard to get the background to work right down at low k, and
it looks like you have plenty of data range based on the number of
independent points.

Second, I don't like forcing S02 to 0.7 because 0.5 is too low very much.
An iffy sample or problems with normalization can both suppress S02 below
the nominally ideal value. Forcing the fit to adopt a higher S02 then just
has the effect of screwing up correlated variables, like the Debye-Waller
factors. I'd look for explanations for the low S02 rather than just
disallowing it. (I certainly have set S02 in some of my published work, but
those were cases of complex fits where it seemed to be a false minimum,
rather than a sample or normalization error.)

Finally, I'd try some "reasonable" constraints on your MS paths. If they
are triangle paths, it is not possible to come up with a rigorous formula
for how to write the debye-waller factors and alpha's for MS paths in terms
of the direct scattering paths, because you don't know for sure how the
motions of nearby atoms correlate. But in my experience, which is mainly
with metals and transition metal oxides, fits are generally not
particularly sensitive to the details of MS path constraint strategies.
Here's a constraint strategy of medium complexity that I teach my
undergrads that you can try:

Def the alpha's of the MS paths to be the average of the alpha's for the
direct scattering paths that make them up, weighted by the length of the
segment. It's quite probable in a material such as yours that some segment
will be a S-S path which you don't have available as a direct scattering
path. Maybe just leave that piece out of the average.

Def the ss's of the MS paths to be the sum of the ss's for the direct
scattering paths that make them up, divided by 4. Why divided by 4? Because
the ss for a half-path is rigorously equal to 1/4 the ss for a whole path.
Why add them? If the ss of each segment is truly uncorrelated, then adding
them is statistically proper. Of course, this is a truly awful
approximation in some cases, notably as the paths come closer to being
focused (i.e. one of the angles of the triangle becomes large). In cases
where the biggest angle in the triangle is more than about 140 degrees, I
just treat it as a focused path, but I doubt that's true for your two MS's.

Please understand that this constraint scheme is fairly arbitrary! When
working with a new material, I suggest trying a constraint scheme like
this, and then seeing how sensitive the fit is to this constraint. For
example, multiply all the ss's of the MS's paths by 2 and see how stable
the fit is. Go back to allowing the alpha for the MS paths to vary freely
and see again if the fit is stable. This kind of probing will tell you
whether the MS paths are something that have to be carefully considered, or
if they are just a small correction to your fit that should be included but
don't have to be included particularly carefully. It is my experience that
the latter is often the case, but you have to find out for yourself on this
material.

--Scott Calvin
Sarah Lawrence College

At 08:20 AM 11/11/2004 -0800, you wrote:
>Hi List,
>
>       It appears that changing the k-weight on my fit does improve the fit. 
>I still have the problem of forcing the sO^2's on the two MS paths to be 
>positive.  Here's some new data with k-weight 3:
>>