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

[Scott Calvin]

Hi Rich,

I know you posted this a while ago, so you may have gotten other 
answers, but I don't see them in my overflowing inbox. :)

Years ago, I did some work with V2O3, and it was a very stubborn 
system to get a good fit on. Other people I knew who were working on 
the system have also had some trouble. This and other evidence led to 
some people publishing speculations on local distortions that get 
averaged out in XRD but show up in EXAFS. I also recall people 
talking about similar difficulties with other vanadium oxide systems. 
(Sorry; I don't have those references handy anymore, but searches 
should locate them).

As far as the second issue you mention, with Ifeffit reversing the 
position of two atoms of the same species, it does happen some times. 
Why not? A V-O path is basically a V-O path to FEFF; it doesn't know 
that one's a single bond and one's a double bond. But since moving a 
path by more than about 0.1 Angstrom in Ifeffit starts to reduce the 
accuracy of the FEFF calculation, it's something you do want to try 
to prevent. One way to do that is with restraints; it shouldn't take 
much incentive for Ifeffit to refrain from switching the paths.

--Scott Calvin
Sarah Lawrence College


At 05:02 PM 4/18/2007, you wrote:
>Greetings!
>
>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.