[Ifeffit] amplitude parameter S02 larger than 1

[Bruce Ravel]

On 03/20/2015 12:48 PM, huyanyun at physics.utoronto.ca wrote:
> Thank you. Our group has one copy of your book, I'll read it again after
> my colleague return it to shelf. I still want to continue our discussion
> here:
>
> If we treat S02 as an empirically observed parameter, can I just set
> S02=0.9 or 1.45 and let other parameters to explain the k- and R-
> dependence? Because S02 is not a simplistic parameter which may include
> both theory and experimental effects, I feel that S02 is not necessarily
> to be smaller than 1, although I admit S02 smaller than 1 is more
> defensible as it represents some limitations both in theory model and
> experiment, but I have a series of similar sample and all their S02 will
> be automatically be fitted to 1.45~1.55, not smaller than 1. Could this
> indicate something?
>
> I actually found in my system, when I set S02=0.9 (instead of letting it
> fit to 1.45), other parameter will definitely change but the fitting is
> not terrible, it is still a close fit but important site occupancy
> percentage P% changed a lot.  So how should I compare/select from the
> two fits, one with S02=0.9 and one with S02=1.45 with two scenarios
> showing different results?


Yanyun,

As I recall, you are looking at those bizarre skuttuderite materials
which consist of a metal framework with an enormous gap.  Sitting in
the gap is your absorber atom.  The center point of the gap is, as I
recall, over 3 angstroms away from the nearest vertex of the
framework.  The point I am about to make hinges upon all that being
more or less correct.

Feff drops neutral atoms into the specified lattice positions then
does a rather simple-minded algorithm to overlap the charges and come
up with the radii that are used to compute the muffin tin potentials.
In the case of one of those atoms rattling about inside the cage, I am
skeptical that Feff's model produces a highly reliable set of
scattering potentials.  Probably ain't bad -- as you said in your
first email, your fits look good.  But it probably ain't quite right
either.  As Scott hinted, mistakes in the theory can show up in
surprising with surprising k- or R-dependence, and surprising
amplitude and phase dependence.

I have absolutely no intuition for how Feff might introduce systematic
error into a fit for the physical situation of a nearest neighbor at a
distance of 3 or more angstrom, so I don't know how to "explain away"
an oddly large S02.

That said, I can think of some experiments that /might/ give some
insight.  Pick something simple, like a metal oxide or a metal
sulfide -- something with a cubic structure.  You don't want this
experiment to get to complicated.

   1. Before generating the feff.inp file, make the lattice constant
      nonphysically large such that the near neighbor distance is about
      3 angstroms.

   2. Run Feff and add up all the paths to make a theoretical chi(k)
      spectrum for your nonphysically large crystal.  For a later
      iteration of this, you might add some synthetic noise to the
      spectrum.

   3. Treat the chi(k) you just made as your "data".  Import it and the
      normal crystal data into Artemis.  Run Feff on the normal
      crystal.

   4. Use Artemis's single scattering path tool to make a path for the
      first shell scatterer at the distance you used to make your
      theoretical data.

   5. Make a simple first shell, four-parameter fit using that SS path.

Can you make a reasonable looking fit?  With sensible error bars?
What happens with the amplitude?  Is it very large or very small?

Perhaps try the experiment the other way around.  Fit the "normal"
theoretical data with the unphysical Feff calculation.

The point I am driving at that I wonder if you can figure out what
happens to the amplitude in a decent fit when you contrive a situation
with an unusually large first neighbor distance.  If you see a trend
in these "Feff experiments", perhaps that can help you understand the
amplitude in your skuttuderite fits.

Again, I have no intuition about this.  I have no idea if my suggestion
will be fruitful or not.  For that matter, I have no idea if my memory
of your problem is correct.

But maybe this is a brilliant suggestion.  Unlikely, but stranger
things have happened :)

B



-- 
  Bruce Ravel  ------------------------------------ bravel at bnl.gov

  National Institute of Standards and Technology
  Synchrotron Science Group at NSLS-II
  Building 535A
  Upton NY, 11973

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