Hi,
This will probably be the most naïve suggestion: the correlation between
coordination numbers (CN) and Debye-Waller factors (DW) in your metal oxides
may be at least partially solved changing the k weighting in the fit
procedure. You can fit data in the R space for example, by weighting
simultaneously in k1 and k3, as you have probably done. In principle this
should allow you to reduce the covariance between CN's and DW's. Another
approached described in the FEFF manual that worked well for me is to fix
the CN at several values around the optimal region and fit the corresponding
DW for a given k-weight. Next you change the k-weight scheme and repeat the
fit of DW's at fixed CN's. After performing this procedure you will end up
with a grid of DW versus CN at three different k-weightings. Plotting DW vs
CN for each k-weight yields a straight line if you are close to the true
minimum. The intersection of the three lines will form a triangle whose
centroid will correspond to the optimum CN/DW pair. Now that you have
extracted k-weight independent DW factors you might try to model these with
a static plus thermal disorder terms. This approach should work well in the
first shell of metal oxides.
Hugo Carabineiro
----- Original Message -----
From: "Ian James Drake"
Hi,
There is little discussion over the significance of fitted Debye-Waller factors (ss2) found for metal centers in metal oxides of heterogeneous catalysts. I am searching for a better way of determining how reasonable these values are in the fits I get.
1. Are phenomenological models like the correlated Debye model and Einstein model appropriate? It seems that Debye temperatures are hard to find for oxides.
2. Is there an equivalent value for ss2 in x-ray crystallography data that I can refer to for comparison in these materials? What should I look for in reviewing published x-ray crystallographic data?
3. Is the temperature dependence reported for the ss2 for some metals (like Cu and Al) similar to their respective oxides? For example, in Debye-Waller factor calculations reported for Al metal by R.C.G Killean (in J.Phys.F: Metal Phys., v. 4, pg. 1908, 1974), the ss2 changes by a factor of three between 25C and 300C. Would I expect a factor of three increase in crystalline zeolites (AlO4 structural units).
4. Specific to my work: I have studied Al containing oxides at temperatures between 25 and 300 C. I would like to quantify coordination changes by doing EXAFS fitting of the Al K-edge data. The EXAFS was taken at temperature and I observe no change in the broadening of the data. Likewise, the fitting shows no sensitivity in the Debye-Waller factor. It is nearly constant at 0.001 A over the temperatures of interest. Since ss2 and the coordination number (CN) are correlated, I would like a way to bind the error on the fitted CN by modeling real physical changes in the Debye-Waller factors. Do you have any suggestions?
Looking forward to your thoughts.
Ian Drake Graduate Student UC Berkeley _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit