[Ifeffit] Large Enot values

Bruce Ravel bravel at bnl.gov
Tue Jun 8 14:56:19 CDT 2010


Are you sure that you are using a sensible crystal structure to run
feff?  1.7 Angstroms is a suspiciously short Se-O distance.  You will
notice that your fit is also making DeltaR fairly large -- pushing
that distance out to a much more sensible value.

Given the very short Se-O distance in the feff calculation, that could
account for odd value of E0 that you are getting in your fits.

What distance do you expect for the Se-O bond?


On Tuesday 08 June 2010 03:34:55 pm Paul Bingham wrote:
> Hi,
> I have been struggling with a tricky problem and cannot resolve it -
>  hopefully someone out there can help!
> I have collected fluorescence XANES and EXAFS at the Se K-edge and I'm
> currently
> trying to fit the EXAFS (see attached files). The XANES agrees very well
>  with a number of my standards and shows that the vast majority, if not
>  all, of the Se is present in this material (an amorphous oxide-based
>  system) as Se4+.
> I have tried to fit the EXAFS using a single Se-O path from a range of cif
> files
> (the simplest being SeO2). Other ions than O present incorrect bond lengths
>  and I am convinced the first shell is oxygen. I can obtain a decent fit in
>  terms of most parameters, as long as I disregard the consistently high
>  value of Enot. Guidelines indicate that Enot should be less than about
>  5(ish), but I consistently get values of 10-12 when I try to fit my
>  spectra, of which one is present in the attached files.
> I have checked my background subtraction and tried as many different
>  options, changes and tweaks that I know or can find suggested (too many to
>  list here) but I cannot obtain a fit with Enot below about 10 and so I ask
>  my colleagues out there who are more experienced than I with EXAFS - can
>  anyone shed any light on (a) the potential cause/s and (b) any potential
>  solution/s?
> Thanks in advance for your time.
> Best Regards
> Paul Bingham


 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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