[Ifeffit] Help with Artemis fit

[Carlo Segre]

Hi Han Sen:

On Sat, 18 Sep 2010, Han Sen Soo wrote:

>
> My first objective is to get an idea of the coordination number of Ti, 
> and I'm trying to do this in Artemis by using the parameter N as my S0^2 
> and letting the amplitude of S0^2 vary. I'm not sure if this is 
> appropriate?

I think that if you attached the project file it would be easier to 
understand this question and whether you are doing the "right thing".

>
> A problem I am encountering is that the physically reasonable fits look 
> visually unacceptable. I am just trying to fit the first Ti-O shell 
> initially and plan to work on the 2nd shell subsequently. The R space 
> fit for the first shell has 2 peaks that are about 0.3 Å apart at 1.2 
> and 1.5 Å (uncorrected). With only 1 shell of O atoms at a distance I 
> found for similar materials, a reasonable fit was found (including 
> background fitting) but the amplitude of the fit at the 1.5 Å peak is 
> too high while the amplitude at the 1.2 Å peak is too low. I naively 
> tried to improve the fit by adding a 2nd shell at a shorter distance and 
> tried different combinations of distances 0.02 Å apart, but I 
> consistently find that the sigma^2 values for one or both shells become 
> negative. I also realized that the resolution of the data may not be 
> sufficient to distinguish shells that are less than 0.2 Å apart, which 
> is what I am expecting based on data on similar materials. I have 
> attached a pdf image of one of the bad fits.

I think that the biggest problem oyu have is that you are fitting the 
background to remove the big peak at 0.85A.  As you can see from your 
curve, the background extends up into the actual data and is undoubtedly 
skewing your results.  Not only that but the background fitting introduces 
6 additional parameters and thus artificially is making the fit "better". 
I would advise you to stay away from the background fitting and 
instead try to improve the background subtraction to remove the low-R 
peak.  can you tell us what were your settings for the background 
subtraction?

If you can't get rid of the background peak with any other means (there 
are other tips on the IFEFFIT Wiki), you can always just fit the data and 
ignore the low-R peak.  That would be preferable than distorting the fit.

Cheers,

Carlo


-- 
Carlo U. Segre -- Professor of Physics
Associate Dean for Graduate Admissions, Graduate College
Illinois Institute of Technology
Voice: 312.567.3498            Fax: 312.567.3494
segre at iit.edu   http://www.iit.edu/~segre   segre at debian.org