Academically speaking, valid, but, practically, may not be useful if your purpose is to fit the data and not just to construct a valid theoretical calculation.

Most likely, the constraints you will have to apply in order to limit the number of variables will be too restrictive which will defeat the purpose of making such a detailed model.

Anatoly 

Sent from my mobile phone, please forgive typos

----- Reply message -----
From: "Douglas Langie da Silva" <douglaslangie@gmail.com>
To: "XAFS Analysis using Ifeffit" <ifeffit@millenia.cars.aps.anl.gov>
Subject: [Ifeffit] Inequivalent atomic positions (V2O5 xerogel)
Date: Tue, Mar 18, 2014 11:22 AM

Thanks guys. With respect to the numbers, I only try a quick fit in order to formulate the question in the list. My interest is to know if the procedure to consider the two inequivalent positions is valid.

Regards.  

2014-03-18 11:57 GMT-03:00 Bruce Ravel <bravel@bnl.gov>:

On 03/18/2014 10:31 AM, Douglas Langie da Silva wrote:

I am working with EXAFS data of V2O5 xerogel. In this form, the V2O5
present a monoclinic structure formed by bi-layers of VO5 pyramids
intercalated with water molecules. Fig 1 attached presents a view of the
V2O5 xerogel structure. There are two Vanadium atoms per unit cell, each
one surrounded by oxygen atoms at distinct distances, characterizing two
inequivalent sites.
My question is how to fit the data in dartemis considering the two
inequivalent positions?

My first approach was run Feff two times. Then I get two theoretical
models, one for V1 atoms and the other for the V2 atoms. Each of Feff
runs are present attached as model V1 and model V2. After that I
transfer the paths of the first shell of each model to dartesmis data
windows (fig 2) and proceed with the fit. We pay attention in the site
fraction of each vanadium atom in order to setup the S02 parameter for
each model. Without attempting at this point with the numbers, the final
results are present in Fig. 3 e 4. Fig 4 presents the log file and the
results for the each patch.

My question is: this procedure is valid?

What you have done seems very much like the common explanation for this given by me and others on this list.

I'd be a bit worried about the negative sigma^2 and I'd wonder why you are not floating Delta R parameters, but those are details.  The basic answer to your question is, seems valid to me.

B


--
 Bruce Ravel  ------------------------------------ bravel@bnl.gov

 National Institute of Standards and Technology
 Synchrotron Science Group at NSLS --- Beamlines U7A, X24A, X23A2
 Building 535A
 Upton NY, 11973

 Homepage:    http://xafs.org/BruceRavel
 Software:    https://github.com/bruceravel

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--
Douglas Langie da Silva
Instituto de Física e Matemática
Departamento de Física
Universidade Federal de Pelotas
Campus Universiário s/n
CP 354
Pelotas/RS
CEP 96010-900
55 53 32757477
55 53 32757345

Douglas Langie da Silva
Physics Department
Federal University of Pelotas
POBox 354
Pelotas/RS
ZCode 96010-900
55 53 32757477
55 53 32757345

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