[Ifeffit] How to analyze this data

Bruce Ravel bravel at bnl.gov
Tue Aug 10 14:02:03 CDT 2010

On Tuesday 10 August 2010 01:54:01 pm Elsa E. Sileo wrote:
> I have some measurements at the As edge of As(V)
> adsorbed onto pure goethite (alfa-FeOOH), Mn
> partially substituted Goethite (alfa-Mn,Fe.OOH),
> Al substituted goethite (alfa-Al,FeOOH) and Mn
> and Al goethite (alfa-Al,Mn,FeOOH)
> The EXAFS signals change from one spectrum to another.
> I have refined the distances As-Fe on pure
> goethite, but when I try to refine Mn-FeOOH or
> Al,FeOOH, the analysis gets more difficult
> because As forms different complexes with Al and Fe.
> Unfortunately for me is almost impossible to have
> the pure phases alfa-MnOOH or alfa-AlOOH, and measured the As adsorbed.
> How can I analyze the spectra and have some
> knowledge about the As-surface complexes formed?

Hi Elsa,

The general problem of analysis of impure phases is one that many
peple have used Feff and Ifeffit on.  Among the folks who regularly
post here on the mailing list, both Shelly Kelly and Scott Calvin have
published on analysis of mixed phase systems and have discussed their
strategies for doing so.  Of course, neither of them has worked on
quite the same system as you have, but the concepts in their papers
are readily transferable.

The basic idea is that, although you may not have pure phase MnOOH or
AlOOH *samples*, you can certainly imagine what the structure of those
might be and make a Feff calculation on that imagined structure.  That
will give you a good guess as to what the contribution to the XAS data
of As in one of the other possible environments looks like.

In you fit, you then take paths from the additional Feff calculations
that you can use to model the parts of the data that come from As in
the other environments.  Of course, this can be a difficult -- you
need to figure out a sensible way to parameterize amplitudes, delta R
values, and sigma^2 values for these additional paths. And you must do
so while being mindful of the limits on the information content of
your data.

Do you have a copy of Shelly's recent review article? 

   Kelly, S. D. ; Hesterberg, D. ; Ravel, B. Analysis of soils and
   minerals using X-ray absorption spectroscopy. In Methods of soil
   analysis, Part 5: Mineralogical methods; Ulery, A. L. ; Drees,
   L. R. , Eds.; Soil Science Society of America: Madison, WI, 2008;
   pp 367− 463.

Shelly can probably send you a copy if your library does not have this

The section starting on page 451 under the heading "Determining a
Second Shell Atom Type within Sediments" is quite relevant to your
problem.  I'd recommend reading that (and, frankly, the rest of the
article) quite carefully.



 Bruce Ravel  ------------------------------------ bravel at bnl.gov

 National Institute of Standards and Technology
 Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
 Building 535A
 Upton NY, 11973

 My homepage:    http://xafs.org/BruceRavel
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/

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