[Ifeffit] Debye-Waller factors for metal oxides ?

[Hugo Carabineiro]

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" <idrake at uclink.berkeley.edu>
To: <ifeffit at millenia.cars.aps.anl.gov>
Cc: "Alex Bell" <alexbell at uclink.berkeley.edu>
Sent: Monday, February 07, 2005 7:25 AM
Subject: [Ifeffit] Debye-Waller factors for metal oxides ?


> 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
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