[Ifeffit] a question about background refinement

[Kropf, Arthur Jeremy]

Stanislav,

You mention that the amplitude remains consistent at about 0.3.
However, what is happening to R and E0?  Are these values also
reasonable?  I have found that, on occasion, a negative DWF is telling
me that oxygen may not be the correct nearest neighbor.  In these cases,
R and E0 are always anomalous for the metal-oxygen path.

Jeremy Kropf

Chemical Engineering Division
Argonne National Laboratory

-----Original Message-----
From: ifeffit-bounces at millenia.cars.aps.anl.gov
[mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf Of Stoupin,
Stanislav A.
Sent: Tuesday, April 13, 2004 5:01 AM
To: ifeffit at millenia.cars.aps.anl.gov
Subject: [Ifeffit] a question about background refinement


Hello all,

Recently, I was able to improve fitting results in one of my projects
using background refinement/subtraction in some "strange" way. I have
got reasonable results but I was not totally sure if that was a valid
approach. So, any feedback is appreciated.

Here is the story:

The system I am working on is Ru-Pt fcc alloy in a form of nanoparticles
which may be oxidized. There is a reason to suspect presence of Ru-O
bonds in the system. For the moment I am doing only the first shell
analysis and the model I am using includes a part that models alloy
first shell structure and also Ru-O path.

If I don't use the Ru-O path in my model all the alloy parameters that I
get from the fit are reasonable but the fit works bad in low R region.
In that region there is a broad peak around 1.3A. The rmin parameter
which is responsible for background refinement is set to 1A ( the same
value was used when I did initial background subtraction in Athena).

What I found a long time ago is that the fit could be significantly
improved  if I add the Ru-O path in my model (all alloy parameters were
even better in that case). However, the DWF of Ru-O bond always went
negative (about -0.005). At the same time I got reasonable value for the
Ru-O amplitude which was about 0.3.

I attempted to get rid of the negative DWF value many times. For
example, if I fixed or restrained the DWF to a reasonable value (0.01 is
what I expect) it didn't help.

Finally, I decided to play with background refinement and what I have
done can be described as follows:

1. Set rmin to about 1.5 A
2. Fit with my model which included Ru-O path with fixed
   delr = 0 and DWF = 0.01 ( the Ru-O amplitude was allowed to vary).
   Backgound fitting was turned on.
3. Subtracted the background from the data
4. Moved rmin back to 1A and allowed Ru-O delr to vary (the DWF remained
fixed to 0.01) 5. Fit again with no background fitting

A very decent looking fit came out as a result. (Chi-square was
improved) The Ru-O amplitude was about 0.3 (as expected)
and I had the DWF set to 0.01.

Thanks for you patience if you read it all the way through.
My own impression of what I have done is that it looks very artificial
and the fact that I use AUTOBK two times bothers me a lot. The second
time it is supposed to remove all structural information below 1.5A
(rmin value). However, with this approach I hope to get rid of the
background that dominates the fine structure in the low R region (Ru-O
scattering) using some preliminary knowledge about the compound.

I will be happy to hear any point of view. Thank you in advance. If you
are interested and want more details please let me know.


Stanislav

--------------------------------
Stanislav Stoupin 
Research Assistant/PhD student 
BCPS Department


Illinois Institute Of Technology 

E-mail:stousta at iit.edu

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