My observation (not rigorously tested though): the longer it takes to get a
good fit, the more suspicious are the results!


-----Original Message-----
[mailto:[EMAIL PROTECTED] Behalf Of Richard
Sent: Wednesday, May 02, 2007 8:56 AM
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] large Eo's and neg. D-W factors


Thanks for the clues!  The restraints seem to be working to get the DW
factors positive.  I'm still battling Ifeffit reversing the paths, but
things are getting slowly better.


> 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:
>>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 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.
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