[Ifeffit] Interatomic distances from feff paths

30 Sep 2010

      I am trying to use the following CIF files to
generate the feff path at the As edge:

1-  'Iron(III) arsenate(V) dihydrate', Scorodite,
H4 As Fe O6'; space group P c a b; number 61; orthorhombic

and

2.  Iron(III) tris(dihydrogenarsenate) pentahydrate

Kaatialaite, Fe (H2 As O4)3 (H2 O)5, P 1 21/n 1, number 14; monoclinic

and in both cases, after running Feff, I obtain
distances Fe-As that are not real.
For instance in scorodite I find a distance As-Fe = 1,71 A.
How can I overcome this problem?

Thanks,

Elsa

At 14:00 30/09/2010, you wrote:
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1. Re: Ifeffit Digest, Vol 91, Issue 28 (Matt Newville)
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Message: 1
Date: Thu, 30 Sep 2010 11:28:28 -0500
From: Matt Newville 
To: XAFS Analysis using Ifeffit 
Subject: Re: [Ifeffit] Ifeffit Digest, Vol 91, Issue 28
Message-ID:

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Hi Elsa,
On Thu, Sep 30, 2010 at 10:33 AM, Elsa Sileo  wrote:
...
Dear Matt,
I have to perform EXAFS analysis, and when I tried to fit the first
shell, I found deltaE0 values of about 8 eV. Following the paper of
Kelly et al (Analysis of soils and minerals using X-ray absorption
spectroscopy. In Methods of soil analysis, Part 5: Mineralogical
methods; 2008; pp 446) I fitted the first shell obtaining values for
deltaE0, sigma^2; delta R and amplitude. Then, using the obtained
values, and making deltaE0=0, I got the theoretical signal.
Why is important to have a theoretical spectra with deltaE0=0?
...
When I compare the experimental and theoretical spectrum I see a mismatch
between the position of the nodes at low wavenumbers.
Yes, that is what E0 does.
...
Having done this, how I have to proceed to correct the experimental
data in order to obtain smalller delta E0 values?
I think what you are trying to do (though I don't understand why) is
to adjust E0 for the experimental data so that the fit to a particular
theoretical standard gives deltaE0 close to zero.  If so, you want to
adjust E0 in the background subtraction.  If you change E0 by 8eV
there, you should end up with a deltaE0 in the fit that is close to
zero.
Hope that helps,
--Matt
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- Release Date: 09/30/10 06:34:00
Dra. Elsa E. Sileo
Solidos Inorganicos
INQUIMAE - Dto. Qca. Inorganica, Analitica y Qca. Fisica; FCEN, UBA
Int. Güiraldes 2160, Pabellón 2, Piso 3, Ciudad Universitaria - C1428EHA
CABA - Argentina
Tel: (54 11) 4576 3380 ext. 113
Fax: (54 11) 4576 3341
e-mail: sileo@qi.fcen.uba.ar