Problems with Reference Paths
Cheers friends, now a short question concerning reference paths calculated by FEFF (I use FEFF8.1): I have a series of spectra of catalysts containing V,P and O. Data was taken at the V K edge. I know from literature, that the first coordination shell should contain some 5-6 O atoms. The problem (and this is really the first time I ever had problems with this) is that ifeffit (V 1.2.1) is not capable of fitting even the first shell. As it works fine for model systems, e.g. Au foil, I wonder if I have problems with the calculation of the references... As first try, I re-calculated the feffxxxx.dat files with the SCF flag turned on (which I never used before for the EXAFS) - the problem persisted. Has one of you experienced similar problems and knows how to come over? Norbert -- Dr. rer. nat. Norbert Weiher (norbertweiher@yahoo.de) Laboratory for Technical Chemistry - ETH Hönggerberg HCI E 117 - 8093 Zürich - Phone: +41 1 63 3 48 32
Hi Norbert, On Fri, 11 Jul 2003, Norbert Weiher wrote:
Cheers friends,
now a short question concerning reference paths calculated by FEFF (I use FEFF8.1):
I have a series of spectra of catalysts containing V,P and O. Data was taken at the V K edge. I know from literature, that the first coordination shell should contain some 5-6 O atoms. The problem (and this is really the first time I ever had problems with this) is that ifeffit (V 1.2.1) is not capable of fitting even the first shell. As it works fine for model systems, e.g. Au foil, I wonder if I have problems with the calculation of the references...
As first try, I re-calculated the feffxxxx.dat files with the SCF flag turned on (which I never used before for the EXAFS) - the problem persisted. Has one of you experienced similar problems and knows how to come over?
I'm not sure I understand the trouble you're having. Could you send an a more complete example? You shouldn't need SCF for EXAFS. In fact, I'd recommend trying Feff7 or Feff6 if you're having trouble with Feff8.1 calculations for EXAFS. PS: To cut down on more spam getting to this mailing list, messages from email addresses that are not in the mailing list members are now 'held for approval' (currently by Bruce or me) before being sent to the list. That might mean some delays in posts for messages sent from non-member email addresses. Please don't take it personally!! --Matt
Hi Matt, okay - I attached a chi.dat file with the EXAFS to fit and a FEFF input file with which I did the reference paths. I tried to fit the first shell with the first five paths which came out of the FEFF calculation. The problem was that the algorithm really went somewhere - no matter what paths or path combinations I used in the fit. The system is a VOHPO4 based catalyst and should have simply some 5-6 O atoms in the first shell. A comparison of the FT EXAFS with the literature looks reasonable. Maybe it is a problem of the correct bg subtraction? I really don't know this time. It might be a good idea to first do V2O5 or some similar compound. But unfortunately, I don't have spectra available. So if anyone has done V2O5 or something similar it would be great to hear of your experiences with this compound. One last word to the catalyst sample: These V cats are lousy for EXAFS (lots of similar scatterers in slightly different positions). So it might be that an out of phase problem is the main point here. Thanx for the help :) Norbert -- Dr. rer. nat. Norbert Weiher (norbertweiher@yahoo.de) Laboratory for Technical Chemistry - ETH Hönggerberg HCI E 117 - 8093 Zürich - Phone: +41 1 63 3 48 32
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Norbert Weiher