Hi, Robert,
Thank you very much for the help. Based on the nominal composition, there
should have more than one Pd atoms in the 1st shell and both the R values
you got are reasonable.
I still have some questions about the fitting
1) Which is better, to refine only the first shell or more shells.
The results seems to be quite sensitive on the selection of kmin/kmax
and Rmin/Rmax, I cannot judge when can get reliable results (base on chi?)
2) You used Rmax=2.56, while Rpd is 2.68, not sure if there's
any contradict
3) For the Nni and Npd, you change them manually and check the chi
values during fitting, is that right?
Thanks again
Fuxiang
On Tue, Feb 21, 2017 at 3:30 AM, 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@. 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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