[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

________________________________

This message and its contents including attachments are intended solely for the original recipient. If you are not the intended recipient or have received this message in error, please notify me immediately and delete this message from your computer system. Any unauthorized use or distribution is prohibited. Please consider the environment before printing this email.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Co-Foil-700C-hex.apj
Type: application/octet-stream
Size: 442117 bytes
Desc: Co-Foil-700C-hex.apj
URL: <http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20150315/9af1e90d/attachment.obj>
```

More information about the Ifeffit mailing list