-----Original Message----- From: ifeffit-bounces@millenia.cars.aps.anl.gov [mailto:ifeffit- bounces@millenia.cars.aps.anl.gov] On Behalf Of John J. Rehr Sent: Monday, February 07, 2005 10:07 AM To: XAFS Analysis using Ifeffit Cc: John J. Rehr Subject: Re: [Ifeffit] Debye-Waller factors for metal oxides ?

The lack of accurate Debye-Waller factors in complex materials,

metal oxides, can be a real problem. In my view, simple

methods like correlated Debye and Einstein models can't always be

I agree with John Rehr. With a temperature dependent data set, the best model that I have found is to allow each atom pair to have a thermal and static disorder component. Sigma2 = debye(Temp,Td_O1) + stat_O1 (This example is for the first oxygen (O1) shell of the metal oxide) The static disorder component should be the same for all data sets from the same sample at the different temperatures. The thermal component should increase as the temperature increases. If you are lucky a Debye or Einstein model will work well enough. You will have to test both. In addition, the thermal change in sigma2 should be about the same for different samples that are closely related. (We could argue about this last point. The best thing to do is to test this constraint by first not imposing it and determine within the uncertainties in the measurement, if it is true. I have found that it usually is an okay constraint.) In the end, you simultaneously model the temperature dependence and the sample dependence spectra all together. You constrain the static disorder term to be independent of temperature, and you constrain the debye temperature to be independent of sample change. This should impose enough constraints on sigma2 values to accurately look at changes in coordination number. Of course the coordination numbers do not change as a function of temperature but only as a function of sample changes. HTH Shelly Shelly Kelly ---------------------------- skelly@anl.gov Bldg 203, RM E113 phone: (1) 630-252-7376 Argonne National Laboratory fax: (1) 630-252-9793 Argonne, IL 60439 www.mesg.anl.gov like phenomenological trusted.

There is an alternative approach to fitting DW factors in such

which might be useful. The idea is to fit a few spring constants k_ij between the atoms, rather than the DW factors themselves, e.g., for the most important sites. This is efficient since the spring constants are temperture independent. This has been tested by Rossner et al. using their Bayesian approach [Rossner, H. H. and Krappe, H. J., (2004). Phys. Rev. B 70, 104102] The fitting is does via an analytical formula for the Debye-Waller factors based on a fast recursion method [Poiarkova, A. V. & Rehr, J. J. (1998). Phys. Rev. B 59, 948].

Of course, such an approach still misses contributions to the DW factors due to structural disorder, which are addititive.

J. Rehr

On Sun, 6 Feb 2005, Ian James Drake wrote:

...

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

(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

...

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

systems metals three 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

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