[Ifeffit] large Eo's and neg. D-W factors

Richard Mayes mayes at ion.chem.utk.edu
Wed Apr 18 16:02:57 CDT 2007


I have inherited a vanadium EXAFS data set (due to a group member 
graduating - lucky dog!) that is giving me major fits.  I am getting 
consistent numbers with all my fits which visually look fantastic, that 
is until I look at the numbers...one Debye-Waller factor is negative (a 
V-O path) and the Eo's are large, the V-Cl path is on the order of -20 
eV, the V=O path is on the order of 17 eV, and the V-O path falling at 
-11 eV.  ARe there any "tricks" outside of restraints or constraints 
that I can attempt to get more realistic values?  (I understand that a 
large Eo can be realistic, but a negative Debye-Waller factor doesn't 
make sense.)  I can restrain the V-O Debye-Waller factor and get a 
positive value, but then the V=O D-W factor is negative.  If I restrain 
both D-W factors, the fit falls apart, suggesting something else is 
wrong that I just can't find.

Another issue I'm having involves something in Artemis that's confusing 
me.  Sometimes after a fit, the Artemis Pallet shows the V=O feature as 
the one at 1.8 Angstroms and the V-O feature at 1.6 Angstroms, 
completely reversed to the FEFF model.  I have seen this when I 
separated the chlorides as well (I attempted to model the Cl at 
different distances as reported by Deguns, et. al. (attached) due to the 
immediate environment around the V in our sample being similar to the 
one they report in scheme 1-(I), p5008).  Attached is a text file that 
shows an Artemis Pallet output illustrating what I'm trying to describe 
for the V=O and V-O paths.  The Artemis project that generated the 
output is also attached.

At this point, I'm wandering what I'm missing.  Any help is greatly 

Many thanks in advance!

Rich Mayes

Barnes Group
450/452 Buehler Hall
Dept. of Chemistry
Univ. of Tennessee
Knoxville, TN 37996
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