Moving the Fe atom off-center as if it were the Ti atom in a perovskite isn't a
good way to simulate JT distortion.
In Fe3+ compounds such as goethite (I assume Fe3+, not the rare Fe4+), the
distortion consists of the two axial
distances being shorter than the 4 equatorial. You could simulate the 1st
shell by moving two of the O atoms
artificially. Alternatively, in Artemis, you could clone the Fe-O 1NN path and
set a parameter for the JT distortion,
but that wouldn't get the MS or higher neighbors correct. Does anyone know of
a program which would be like ATOMS,
except with parameters, thus generating parameterized distances?
The beat you get from the JT distortion will make a big difference in the fit
quality once you get to something
like 14A^-1.
mam
On 8/8/2011 6:21 PM, Matt Newville wrote:
Hi Enyuan,
On Mon, Aug 8, 2011 at 7:47 PM, Enyuan Hu<bearcha...@gmail.com> wrote:
Dear all,
I'm trying to fit the Fe K-edge EXAFS data and I appreciate that Fe in my
sample mainly exists as Fe4+, which is associated with Jahn-Teller
distortion. The one shell model that considers Fe residing in the standard
octahedral apparently didn't give satisfactory result. I'm sorry that even
though I found people do fitting considering Jahn-Teller distortion, I am
still not aware of how to carry that out in Artemis. Can someone helps me to
look at my fitting and give me some guidance on how to implement fitting
that considers Jahn-Teller distortion in Artemis? Thank you.
I noticed a few things that might lead you to thinking about the problem better:
You started with the crystal structure for LiCoO3, which has
octahedral symmetry, and calculated for the Co edge, even though your
data is for Fe edge (and one can see the Co K edge showing in the
chi(k) data at ~12.5 Ang^-1).
What you want to do is *edit* the feff.inp that results from running
Atoms on the crystallographic data. When doing, you will definitely
want to make the central atom Fe, by changing
POTENTIALS
* ipot Z element
0 27 Co
1 3 Li
2 27 Co
3 8 O
to
POTENTIALS
* ipot Z element
0 26 Fe
1 3 Li
2 27 Co
3 8 O
While you're in there messing with feff.inp, you can introduce your
own hypothetical Jahn-Teller distortion, by moving the absorbing atom
away from 0, 0, 0, from
ATOMS * this list contains 103 atoms
* x y z ipot tag distance
0.00000 0.00000 0.00000 0 Co 0.00000
to perhaps
ATOMS * this list contains 103 atoms
* x y z ipot tag distance
0.00 0.04 0.00 0 Fe 0.00000
(though I'm not at all sure I have the direction for the distortion
correct in the R -3 m structure). That will lead to a splitting of
the Fe-O distances, for the distortion you're expecting.
I also guessed at the size of that distortion. Since the resolving
power of EXAFS (especially with data only out to 12.5Ang^-1) is
limited, so you probably will have a difficult time distinguishing the
split in distance from an increased sigma2. That is, you may hae
trouble proving that there is a Jahn-Teller distortion from the EXAFS
data alone.
My final pieces of advice would be: don't use dk=1 with the
Kaiser-Bessel window (it makes an otherwise ideal window function
horrible) -- use dk=4 or dk=5.
Hope that helps,
--Matt
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