[Ifeffit] Debye function model, Math expression

NIRAWAT THAMMAJAK n.thammajak at gmail.com
Fri May 11 11:26:42 CDT 2012


May I add more questions to this. I do a research on magnetic materials.
Bond lenghts and geometries are really important to understand the magnetic
interactions and I think EXAFS is the promising tool for this. I have
measured the EXAFS spectra for quite a complex material system. There are 3
sites of absorbing atom (Fe) in the crystal structure, therefore I created
3 FEFF models to fit the data with different amplitudes. Closest neighbours
are oxygen atoms, and then Li. The strongest scatterers are La atoms
sitting ~4.26 angtrom away from the absorbing atom, which contribute a
significant 3rd shell. Yes, there are many paths to include. Math
expressions together with bulk structure details obtained from neutron
diffraction help to minimise the parameters somewhat. However, there are
thermal factorss, very small but too many of them.

I measured my sample at 1.5 K, then sigma^2 are quite small. Could you
please suggest any papers, models, or math expressions constraining these
factors at low temperature. Would it be sensible to constrain them as 0, or
a range of small numbers obtained from the significant shells?

Thank you very much,

On 24 August 2010 18:25, Frenkel, Anatoly <frenkel at bnl.gov> wrote:

>  Just to add:
> These procedures, described in that article, can be easily implemented in
> Mathematica.
> Anatoly
> ------------------------------
> *From:* ifeffit-bounces at millenia.cars.aps.anl.gov on behalf of Ravel,
> Bruce
> *Sent:* Tue 8/24/2010 1:23 PM
> *To:* XAFS Analysis using Ifeffit
> *Subject:* Re: [Ifeffit] debye function
>  On Tuesday 24 August 2010 12:23:18 pm Kleber Daum Machado wrote:
> >   Hi,
> >
> > I'd like to know the explicit mathematical expressions that are
> > represented by the functions debye(temp,thetad) and eins(temp,thetae).
> > Are they given in some documentation file or article?
> See
> "Extended x-ray absorption fine structure Debye-Waller
> factors. I. Monatomic crystals"
> E. Sevillano, H. Meuth, J. J. Rehr
> Phys. Rev. B 20, 4908–4911 (1979)
> http://link.aps.org/doi/10.1103/PhysRevB.20.4908
> B
> --
>  Bruce Ravel  ------------------------------------ bravel at bnl.gov
>  National Institute of Standards and Technology
>  Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
>  Building 535A
>  Upton NY, 11973
>  My homepage:    http://xafs.org/BruceRavel
>  EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/
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