Re: [Ifeffit] Fe K-edge second shell problems

2011-08-03 Thread Andrea L Foster
Paul, have you considered explicitly multiple scattering from within the 
tetrahedron?  The short distance you describe for the second shell might 
be consistent with that.

Its pretty easy to write an expression in the fit routine that makes the 
MS path distances explicitly dependent on the SS paths (no new variables 
in the fit)--the number of MS paths is constrained by the geometry 
(tetrahedral).  The order of scattering is probably just 3- and 4- legged 
MS paths, no higher (would give longer distances)

I study arsenic, and so I've seen this approach described in the following 
paper on As: 

Voegelin et al (2007) Geochimica et Cosmochimica Acta v. 71, pp. 
5804-5820.  Look in the section 2.6 for the details.

sorry if someone already answered this,

Andrea L. Foster, Ph.D.
345 Middlfield Rd., MS 901
Menlo Park, CA 94025
T: 650-329-5437
F: 650-329-5491

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Re: [Ifeffit] Fe K-edge second shell problems

2011-08-03 Thread Scott Calvin

Hi Paul,

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!

--Scott Calvin
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.

Warm Regards

Paul Bingham

Dr. Paul A. Bingham
Immobilisation Science Laboratory
Dept. of Engineering Materials
University of Sheffield
Mappin Street
S1 3JD

Direct Line: (0114) 2225473

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