[Ifeffit] Fitting EXAFS with out any atomic coordinates?

Anatoly Frenkel frenkel at bnl.gov
Wed Mar 28 15:43:35 CDT 2007

Since I like the sound of my voice too, I will translate the "zeitgeist"
from Bruce's email. According to Wikipedia, "It is originally a German
expression that means "the spirit (Geist) of the time (Zeit)". Can it be
loosely translate here as "poltergeist"?


-----Original Message-----
From: ifeffit-bounces at millenia.cars.aps.anl.gov
[mailto:ifeffit-bounces at millenia.cars.aps.anl.gov] On Behalf Of Bruce Ravel
Sent: Wednesday, March 28, 2007 3:41 PM
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] Fitting EXAFS with out any atomic coordinates?

On Wednesday 28 March 2007 13:15, Mario Gomez wrote:
> Hi Bruce my name is Mario and I was just wondering if your program can fir
> the EXAFS spectra without any atomic positions. Obviously the answer is no
> if you use atoms to create the FEFF file but what does one do when we have
> a semi-crystalline material for which no one has attempted the fractional
> coordinates.
> I mean the XRD pattern shows some crystalline but due to the high
> background I would think it would be very hard to do the Rietveld
> So what can one do, can we fit the EXAFS spectra knowing only the chemical
> composition and perhaps the lattice geometry(orthorhombic or hexagonal)?


Scott's answer was excellent, but I like the sound of my own voice so
I thought I'd answer as well ;-)

The Feff+Ifeffit+Artemis zeitgeist requires a list of atomic
coordinates to begin analysis.  Except in the rare situations, we
don't know the actual arrangement of atoms.  Indeed, to find out is
one of the reasons we might do EXAFS.

As Scott suggested, thinking about what you sample resembles is
usually a good place to start.  Consider amorphous germanium as an
example.  aGe is typically described as a continuous random network of
bonds which are about the same length as the bonds in crystalline Ge.
Because of that, crystalline Ge is a good place to start.  In fact, I
suspect that you could make considerable progress in analyzing aGe
data with that one Feff calculation on the well-ordered material.

EXAFS analysis is something of a creative endeavor -- the trick is to
figure out how to use things that you know to model things that you
don't know.  Reading papers by the folks who answer questions on this
list (Scott Calvin, Shelly Kelly, Anatoly Frenkel, Paul Fons) is a
really good source of ideas for your own work with Ifeffit&Artemis.


 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

 Molecular Environmental Science Group, Building 203, Room E-165
 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007

 Argonne National Laboratory         phone and voice mail: (1) 630 252 5033
 Argonne IL 60439, USA                                fax: (1) 630 252 9793

 My homepage:    http://cars9.uchicago.edu/~ravel 
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/

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