[Ifeffit] Using more than one E0

Kelly, Shelly D. SKelly at anl.gov
Wed Dec 15 10:05:41 CST 2004


Hi Scott,

See below:

The MS scheme you outline below seems to treat the E0 due to the
scattering species (I'll call that the "bonus" E0 for lack of a better
term) as being the average of the bonus E0's for all the scattering
atoms
in the path. Thus you give the Cu-O1-Cu1-Cu path as using the average of
the O1 and the Cu1 bonus E0's, while the Cu-O1-Cu-O1 path uses just the
O1 bonus E0. I would naively expect we should be using the sum, rather
than the average. After all, if this bonus E0 is a correction to the
shift caused by the muffin-tin potential FEFF has calculated, shouldn't
a
MS path that bounces off the same atom twice have to use twice the
correction? I feel like there's something basic I'm missing here...

[Kelly, Shelly D.] The energy shift applies to the entire scattering
event.  The scattering event can not be broken down into the sum of the
legs.  Hence the energy shift for the entire scattering event can be
approximated as an average of the energy shift from each leg.  I have
used the average energy shift a lot and found that it works well.  I
have never tried using a sum of the energy shifts...   I think that in
an extreme limit you can convince yourself that the sum will break down
for an 8 legged paths.  The energy shifts are "never" greater than 10
eV.  "never" in my experience.  -Hence the answer to your next question.

Also, what is your experience as to the actual differences in bonus E0's
that you get in ionic compounds? Right now one of my students is working
with Sr(NO3)2... should we really expect the difference in bonus E0's
between the Sr2+ and the oxygen atoms to be more than 12 eV, or does
some
kind of charge overlap from the nitrate ions actually make the strontium
ions behave as if they are less than +2?

[Kelly, Shelly D.] One full electron of ionic bonding gives about 6 eV.
So that is about the most you would expect from different ions in a
solid.  Usually, I have found 3 to 4 eV.  You make up most of the
valence state change from the shift in the edge position.  I have found
unrealistic energy shifts when I try to use too much chi(k) data at low
k.  I'd look there first for you 12 eV shift.

HTH
Shelly





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