Hi Bruce,
Thank you for your insight. The current materials I am talking about
is something a little different. The first-shell distance for one site
is about 2.5 angstrom, and for the other site is 3.3 angstrom.
I feel happy that you still remember that bizarre system which I am
still working on. Your brought up a very interesting test to do. I
will definitely do this test experiment to see what would happen to
the amplitude S02 if the first-shell distance is larger than 3
angstrom. This might take me for a while. I will come back to you
after I do it.
Best,
Yanyun
Quoting Bruce Ravel <bra...@bnl.gov>:
On 03/20/2015 12:48 PM, huyan...@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 ------------------------------------ bra...@bnl.gov
National Institute of Standards and Technology
Synchrotron Science Group at NSLS-II
Building 535A
Upton NY, 11973
Homepage: http://bruceravel.github.io/home/
Software: https://github.com/bruceravel
Demeter: http://bruceravel.github.io/demeter/
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