[Ifeffit] Evaluation of uncertainty using correlated Debye model + advices for the fitting of an hexagonal Co-foil

Samy OuldChikh samy.ouldchikh at kaust.edu.sa
Sun Mar 15 08:50:10 CDT 2015

Dear all,

I would like to go beyond the fitting of the first shell for a cobalt metallic foil acquired at FAME beamlime (ESRF).
It's not mandatory for the determination of So in this case, but I just would like to improve myself in EXAFS fitting.

The cobalt foil has an hexagonal structure (P 63/mmc, a =b=2.507 A, c= 4.068 A) according to the supplier.
The first 49 scattering paths calculated with FEFF were selected to cover the R range up to 5.1 A. This include multiple scattering paths.
The parametrization of those scattering paths is as follow (.apj enclosed) :

- Distances :
1) a spherical expansion term was used to describe distance variation in [hkl] direction (reff*alpha1) with l considered as "large"  when compared to h,k (reff*alpha1)
2) another spherical expansion term was used to describe distance variation in pure [hk0] direction (reff*alpha2)
3) a rough average of two previous expansion terms for [hkl] direction when h,k are comparable to l (reff*(alpha2+alpha1)/2).

- For modeling disorder, the correlated Debye model was used with 3 parameters dt, dt2, dt3 (t =293K):
1) [hkl] direction with l considered as "large" : debye(t,dt)
2) pure [hk0] direction: debye(t,dt2)
3)  for [hkl] direction with h,k comparable to l : debye(t,(dt+dt2)/2)

But does this choice of parametrization is really reasonable?
It seems very rough to me but when I look at the fit results it looks ok.

- Uncertainty on :

1) Distances :  is it simply delta(r) = reff*delta(alpha) ?
2) Debye-Waller : I can't figure out how to relate the uncertainty computed for the Debye temperatures (dt, dt2) to the uncertainties of the debye-waller terms for each scattering paths. Although I spent some times trying to understand this paper, http://journals.aps.org/prb/pdf/10.1103/PhysRevB.20.4908, I can't write a simple relation at the end.

Many thanks for your kind help,

Best Regards,
Samy Ould-Chikh

KAUST Catalysis Center
Bldg.3,Level 4, #4231
4700 King Abdullah University of Science & Technology
Thuwal 23955-6900
Kingdom of Saudi Arabia

Tel: +966 12 8084486
E-mail: samy.ouldchikh at kaust.edu.sa
Website:  http://kcc.kaust.edu.sa/Pages/Home.aspx


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