[Ifeffit] Need an advice

Matt Newville newville at cars.uchicago.edu
Fri May 6 15:06:28 CDT 2005


Hi Steven,

On Thu, 5 May 2005, Steven S. Lim wrote:

> Dear all,
> 
> I have a question to ask all of you as follows.
> 
> We have been investigating the EXAFS of some partially reduced
> Co materials (by Co EXFAS) and would wish to obtain some
> qualitative information on the Co metal particles. If, in
> fact, the Co had some average coordination environment because
> we have a distribution of Co particle sizes with oxygen
> adsorption, analyzing the windowed Co-O and Co-Co peaks
> together as a linear combination and assessing the average
> oxygen and Co coordination from the Co edge absorption (what
> we do with Athena) would appear to give a useful answer.
> However, if what we have is closer to a physical mixture of
> small Co metal particles and Co oxide particles, it would seem
> that it would be more appropriate to analyze separately the
> windowed Co-O and Co-Co peaks in R-space, to somehow normalize
> the Co-Co to the fraction of metal in the sample and use this
> as a qualitative measure of metal particle size. Is there a
> way to do this with Athena or other method? Or do you know of
> a reference where this has been discussed? Of course, we can
> (and will) make experimental mixtures as references but wonder
> if there is a different way of doing the analysis or if there
> is some literature on this problem (which we have not been
> able to locate).

If I understand correctly, you are asking where there is way to 
distinguish a two possible cases:
  1) a physical mixture of metallic cobalt + cobalt oxide  
     (what I would call "mixed phases") ; and 
  2) a chemically intermediate phase, that was neither 
     fully metallic nor oxidized.

I don't know enough about the system you're working on to know
the exact details.  In general the EXAFS analysis (as with
Artemis) allows one to distinguish these case, though it can
sometimes be complicated.

For the first case, you could create models of Co-Co and Co-O
and fit the spectrum as a linear combination of those two phases
without allowing the bond distances to change from the known
values for metal and oxide phases.  That gives one fit, with a
result for metal / oxide fraction.

For the second case, you could allow both Co-Co and Co-O
contributions but allow the distances and coordination numbers
to vary.  If the distances move significantly from those of the
isolated phases, that's good evidence that a simple physical
mixture is not sufficient.  But also, you can compare the
goodness-of-fit parameters for the two cases to help decide
which of the two cases better explain the data.

Ideally, you could also include further "shells"  in this
analysis, especially for second neighbor Co-Co in CoO.  This is
more work, generally requires fairly good EXAFS data, but can
really improve the confidence that you have a component that is
the isolated oxide.

Hope that's enough to get you started in the right direction.  
If not, let us know!

--Matt




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