[Ifeffit] S02

[Bruce Ravel]

On Thursday 20 October 2005 11:13, Juan Antonio Maciá Agulló wrote:
> Hi people,
>
> I fitted several supported platinum based catalysts by Artemis. I
> obtained good R-factors < 0.02 but stranges values of SO2 (<0.30) and I
> do not know if these low values of SO2 are related with a low
> coordination number for Pt. I know that particle sizes of these
> catalysts are very small and then the coordination number is smaller
> than 12.

Without knowing anyting else about the system, I am inclined to agree
with Anatoly and you that the low S02 is indicative of small particle
size and the effective reduction of coordination.

But it seems prudent to seize upon your question as an opportunity to
remind everyone that interpreting the S02 parameter is one of the
hardest parts of doing exafs analysis.  As Juan Antonio points out,
S02 is heavily correlated with coordination.  It is also heavily
correlated with empirical problems such as sample inhomogeneity.  S02
can also be quite highly correlated with sigma^2.  Often times a
suspiciously small S02 is coincident with a suspiciously small
sigma^2.  There are even situations where you might be suspicious of
Feff's energy model.

In any case you should always evaluate a fitting result in the context
of what you know about the sample.  In the case of a supported
platinum catalyst, you have good reason to suspect the coordination
number is effectively reduce compared to the bulk.  One should always
be suspicious of sample preparation protocols and do whatever is
reasonable within the context of the experiment to make the sample a
homogenous as possible, remembering that an absorption length in your
material might be only a few microns.  Finally, one should always be
mindful of correlations with other variables -- often a suspicious
value for a parameter is Artemis's way of telling you that you need to
re-evaluate the decisions you made when constructing your fitting
model. 

But, if you can explain the suspicious value you get for a parameter
in the context of what you know about the sample, then you might be 
justified in running with it.

B

-- 
 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

 Molecular Environmental Science Group, Building 203, Room E-165
 MRCAT, Sector 10, Advance Photon Source, Building 433, Room B007

 Argonne National Laboratory         phone and voice mail: (1) 630 252 5033
 Argonne IL 60439, USA                                fax: (1) 630 252 9793

 My homepage:    http://cars9.uchicago.edu/~ravel 
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/