Looking at your data, I agree that there does seem to be second-shell
scattering present in the signal.
Unfortunately, that most likely means that your material is not close
to one of the "obvious" model compounds. One possibility to keep in
mind is that you may have some kind of overlapping paths in that
second "shell"--for example, partially Fe-Na and partially Fe-Ca, or
an assortment of Fe-Na's at different distances.
A clue can perhaps be obtained by noting the relative height of the
peak near 2.3 angstroms compared with the large peak you've fit. As k-
weight is raised from 0 to 1 to 2 to 3, the peak at 2.3 angstroms
does not grow relative to the first peak. That suggests the scattering
may be from another low-Z element like oxygen.
So I'd tentatively try an Fe-O path around 2.7 angstroms with its own
delR, ss, and N guessed. (Meanwhile, set N_1 to 4 to reduce
correlations--you've said you expect the first shell to be tetrahedral.)
Good luck--sounds like a stubborn one!
Sarah Lawrence College
On Aug 3, 2011, at 9:49 AM, Paul A Bingham wrote:
Dear Ifeffit users,
I have been struggling with this problem on and off for many months
and I cannot resolve it - hopefully someone out there can help....
I have collected fluorescence Fe K-edge EXAFS of oxide glasses doped
with low levels (0.2%) of Fe. The glasses are typified by their major
components SiO2, Na2O, CaO and also low levels of Fe2O3 and CeO2
dopants. I'm currently trying to fit the Fe EXAFS. The first shell is
relatively easy to fit and I'm reasonably happy with the fit I
obtained using a tetrahedral Fe3+ standard, in this case FePO4. The
fits are consistent, as I expected to find, with Fe3+
tetrahedrally-coordinated with four oxygens.
The problem comes - and here's where I could really use some
suggestions - when I try to fit second Fe-x distance. It seems clear
to me that a second Fe-x distance (and possibly a third) are present
in the data. However, despite expending a great deal of time I am
unable to get a fit that appears anywhere near sensible and robust and
for which the output parameters are sensible. I suspect the second
Fe-x distance (I reckon about 2.8 Angstroms) to be Fe-Na but Fe-Ca,
Fe-Si or Fe-O may also be possible. It's also possible that it is
Fe-Fe or Fe-Ce.
I have tried all of the "obvious" Fe model compounds (aegirine,
clinopyroxine, etc) and also many others and I simply cannot get
anything approaching a decent fit. The vast majority of distances in
model compounds are Fe-O distances around 1.9-2.1A, then there is
usually a "gap" until about 3.1A.
I have checked my background subtraction and tried out many different
options, changes and tweaks that I know or can find suggested but I
cannot obtain a fit that is any good. And so I ask my colleagues out
there who are more experienced than I with EXAFS - can anyone help
with this conundrum?
I have attached the Artemis file with the data and simple one-shell
fit using FePO4 cif file; and the Athena file FYI.
Thanks in advance for your time and I look forward very much to
reading any suggestions you may have.
Dr. Paul A. Bingham
Immobilisation Science Laboratory
Dept. of Engineering Materials
University of Sheffield
Direct Line: (0114) 2225473
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