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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