[Ifeffit] Bond length and Coordination number problems from EXAFS

[Robert Gordon]

Hi Fuxiang,

I tried fitting just the first shell in your data:
kmin 3.5  kmax 11.775
Rmin 1.25  Rmax 2.56

I used just Pd-Ni and Pd-Pd single scatter paths with one amplitude, one 
E, 2 delR's and 2 sigma-squareds
for a 6 parameter fit.

I didn't let Nni or Npd vary in a fit, but did change them between fits. 
I get what seems to be
a double minimum yielding similar chi values as I varied Nni+Npd = 12
i.e. Npd in the range 1.6 - 1.8   or Npd 2.0 - 2.2 yielded similar values
Another minimum occurs near 2.4 but sig^2-pd is getting larger than 
seems plausible.

I did not try changing Ranges or windows to see if that would smooth out 
the oscillations in the fit
versus relative N values

amp ~ 0.7 (ok...but I didn't normalise the data or compare with Pd or Ni 
foils)
delEo  -7 (which seems large)
Rni  2.54            Rpd 2.68   (both a bit small but would need to 
compare to Ni and Pd foil fits for systematic error in fitting)
ssni  0.0050(2)    sspd   0.010(3)   (error bars from correlation with 
changing N - strong correlation
between Npd and sspd)

This is not consistent with your model, since it has more than 1 Pd in 
the 1st shell.
The R-values (apart from not having checked reference foils) are within 
reason for what one
would expect from sums of atomic radii.

Hope this helps get you pointed in the right direction as you refine 
your model.

cheers,
Robert



On 2/20/2017 10:57 AM, Fuxiang Zhang wrote:
> Dear All,
> I met a problem in fitting my sample. The sample is a very 
> simple crystalline Ni (fcc) with minor Pd. I measured both the K-edge 
> of Ni and Pd and fit with a 55 randomly distributed cluster model 
> (from calculations). The model suggested that there are 1 Pd in the 
> first and second-shell, 6-Pd in the 3rd-shell and 2-Pd in the 4th 
> shell. I have attached the FPJ file here, can anyone help me to have a 
> look.
>
> 1. In fitting the K-edge of Pd, most paths are single scattering, but 
> I always get a shorter Pd-Pd length than Pd-Ni, even I tried to 
> include all the paths. Not sure if I made any mistakes in the path 
> parameters settings.
>
> 2. If including the minor contribution paths, I use the same 
> parameters for the path: @Ni1.1 Ni1.1@ (acute triangle) and the path: 
> @N1.1 Ni1.1@ (obtuse triangle), as well as single scattering path 
> @Ni1.1 at . I know there's a problem (though no much effect on the 
> results), but don't know how to set the first two paths' parameters
>
> 3. If the result is correct, which may suggest there is no Pd in the 
> 1st-shell, but how to fit the coordination number of Pd is a problem. 
> Since Pd is only 1/12 in the first shell, while the uncertainty of CN 
> from EXAFS fitting is pretty large. Does it make a sense to refine the CN?
>
> FPJ file is attached
>
> Thank you very much
>
> Fuxiang Zhang
>
>
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