[Ifeffit] Fe oxide

Kelly, Shelly D. SKelly at anl.gov
Mon Sep 26 10:06:53 CDT 2005


Hi Juro,

I had some fun playing with this project file.  I hope you don't mind. 
Here is a list of suggestions.
1st:  Don't use the paths that are less than 0.1 Angstroms apart.
1st-b:  If you don't know the CN set it to ONE and then vary just the
amplitude.  There is a degen box multiplied by a amp box.  Degen should
be one.
2st:  set the values for CN, and s2, while letting delr vary
3rd.  Let the CN and delr vary.  You will find that the Fe path around 3
Ansgroms isn't needed.
4th.  Make sure your fit range and data ranges are appropriate.
5th.  Add as much constraints as you think is reasonable.  First Oxygen
coordination number should be 6 or 4 total.  I thought 6 worked well.
6th.  Fit the data with k-weight of 1,2, and 3, to get the uncertainties
smaller.

Take a look at the project file.  I think that this model or one like it
might work.  You need a separate S02 value from the Amp value.  Amp
should be S02*Amp.  I didn't add a S02 part.  You should get that from a
known standard measured on the same beamline.  S02 should be between
0.70 and 1.0.  So the CN reported are likely a bit too big.  

Good luck,

Shelly

Shelly Kelly                   Bldg 203 RM E113
Skelly at anl.gov              Argonne National Laboratory
630-252-7376                 9700 S Cass Ave
www.mesg.anl.gov         Argonne, IL 60440
 
 
 

> -----Original Message-----
> From: ifeffit-bounces at millenia.cars.aps.anl.gov [mailto:ifeffit-
> bounces at millenia.cars.aps.anl.gov] On Behalf Of Juraj Majzlan
> Sent: Monday, September 26, 2005 8:46 AM
> To: ifeffit at millenia.cars.aps.anl.gov
> Subject: [Ifeffit] Fe oxide
> 
> Hello,
> 
> I am trying to fit an EXAFS spectrum of an X-ray amorphous
> Fe precipitate. In a model (goethite), there are Fe atoms
> at 3.02, 3.29, and 3.45 A. Fitting the two most distant
> shells of iron gives strange results. Fitting them
> simultaneously gives very high amplitudes (32, -66) which
> cancel each other (fig. 1) and produce a beautiful and
> unreasonable fit (fig. 2).
> 
> Fitting them separate gives small ('reasonable')
> amplitudes, but the distance (dr) have very high
> uncertainties, e.g., dr = 0.12 +- 20.3.
> 
> Can I expect to resolve these three Fe shells or is it
> asking too much?
> 
> I am also attaching the Artemis file, but will appreciate
> any comments and suggestions.
> 
> Juro Majzlan
> 
> 
> ----------------------------------
> Juraj Majzlan
> Institute of Mineralogy and Petrology
> Albert-Ludwig-University of Freiburg
> Albertstrasse 23b
> Freiburg, D-79104, Germany
> telephone +49-761-203-6416
> fax +49-761-203-6407
> http://www.minpet.uni-freiburg.de/sites/personen/majzlan.html
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