[Ifeffit] Inequivalent atomic positions (V2O5 xerogel)

[Anatoly I Frenkel]

I think it is not valid. You have too many fitting parameters for the first shell (all your sigma2 are independent variables) and you ran out of independent points that are limited by the delta R and delta K. To increase delta R you extended your fitting range to the second shell but is cheating because your theory does not account for the second peak.

You should try to make sigma2 constant, or try a Debye model, or, better, use the following approach. Make a histogram of all 10 distances that are present in the structure between V and O. See if the histogram is more or less symmetric. If it is, use a representative path that is close to the middle distance from the histogram and assign CN =5, and effective sigma2 (that will include both dynamic and configurational components).
Also. Include delta R for that path.

Then you will have a chance.

Anatoly

Sent from my iPad

> On Mar 18, 2014, at 10:44 AM, "Douglas Langie da Silva" <douglaslangie at gmail.com> wrote:
> 
> Dear all,
> 
> 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?
> 
> Since now thak you.
> 
> --
> 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
> <Rav 1.jpg>
> <Model v1.jpg>
> <Model v2.jpg>
> <Fig 3.jpg>
> <Fig 4.jpg>
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