[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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