[Ifeffit] Vanadium center fitting issues

[Scott Calvin]

Hi Chris,

Yes, you do need to account for the inequivalent vanadiums. Each feff  
calculation needs to be weighted by the fraction of vanadiums of that  
type (if you're looking at a tabulation of the asymmetric unit in a  
crystal structure, the number in the position codes like "8f" next to  
the atoms tell you the relative numbers of each). Use the S02 field to  
apply this weighting.

In one sense, that doubles the number of parameters. But the V-V paths  
are present in both calculations, so that cuts it down a little. From  
there, you have the usual options of trying to apply constraints that  
aren't physically unreasonable, and seeing how the fit responds to them.

--Scott Calvin
Sarah Lawrence College

On Jul 10, 2010, at 11:03 AM, Christopher Allen wrote:

>
> Hi,
>
> I wanted to get some advice on a vanadium centered material I’ve  
> been trying to fit for quite a while w/ repeated failure. Based  on  
> XRD and electrochemistry I’m pretty confident in the material/model,  
> and I have the input file for the material.   The first shell out to  
> 2.2 angstroms in R space is a distorted VO6 w/ bond lengths ranging  
> from 1.6 to 2.2 angstroms, similar to the V2O5 which has been  
> discussed on the mailing list recently (July 7th).  The last two  
> peaks from 2.2 to 3.3 angstroms presumably include 4 V-P and 1 more  
> V-O single scattering paths. (the best fit I could get on the 4V-P/ 
> 1V-O used only the first of these two peaks.)
>
> With regards to the first shell fitting, is it appropriate to be  
> grouping the V-O bonds in terms of 1 short, 4 medium, and 1 long  
> bond distance to cut down on variables?  I noticed in the V2O5  
> previously discussed, that he used one ss and delr term for all 6  V- 
> O paths, but I guess that just means making some assumptions that  
> any variations in ss and delr are isotropic throughout.  Is that  
> correct?
>
> I’ve had real issues w/ those peaks representing V-P/V-O from 2-3.3  
> angstroms so I wonder if it’s unreasonable to try and  extract this  
> information based on the k space or if possible thats not what I  
> have there.   Could it be that I need to account for the two  
> inequivalent V centers?  (In that case, wouldn’t the number of  
> variables be doubled?)
>
> Thanks for any comments,
>
> Chris
>
>
> -- 
> Chris Allen
> Northeastern University Center
> for Renewable Energy Technology
> 317 Egan Research Center
> 360 Huntington Ave.
> Boston, MA 02115
> 617-373-5630
>
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