[Ifeffit] Vanadium center fitting issues

[Christopher Allen]

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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