[Ifeffit] Inequivalent atomic positions (V2O5 xerogel)

[Carlo Segre]

Hi Douglas:

On Tue, 18 Mar 2014, Douglas Langie da Silva 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?
>

This is the right way to fit these inequivalent sites as others have 
mentioned.  The biggest question is what are you trying to learn about 
this material?  Depending on your answer, I would modify the model to 
extract the important information.  The challenge you have in this data is 
that you have 8 paths and a very limited number of free parameters.  It 
looks like you have chosen to allow the E0, S02, and sigma^2 to vary.  A 
different selection of variable parameters might suit your goals better.

Carlo

-- 
Carlo U. Segre -- Duchossois Leadership Professor of Physics
Director, Center for Synchrotron Radiation Research and Instrumentation
Illinois Institute of Technology
Voice: 312.567.3498            Fax: 312.567.3494
segre at iit.edu   http://phys.iit.edu/~segre   segre at debian.org