[Ifeffit] a question about background refinement

Stanislav Stoupin stousta at iit.edu
Tue Apr 13 05:01:08 CDT 2004

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 Stoupin 
Research Assistant/PhD student 
BCPS Department

Illinois Institute Of Technology 

E-mail:stousta at iit.edu

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