[Ifeffit] Inequivalent atomic positions (V2O5 xerogel)

[Bruce Ravel]

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 at bnl.gov

  National Institute of Standards and Technology
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