Scott, 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.
Rich > 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. > > > _______________________________________________ > Ifeffit mailing list > Ifeffit@millenia.cars.aps.anl.gov > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit > _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
