[Ifeffit] Hexagonal WO3 (Carlo Segre)

[Carlo Segre]

Hi Yordy:

I have removed all the previous emails to keep the message short...

On Sat, 22 Nov 2008, Yordy Licea Fonseca wrote:

> Hi Carlo and Ying,
>
> Does it means that if the sample is a random solid solution at experimental
> temperature, in principle, you would be able to simulate the EXAFS spectra
> whichever be the Mo/W ratio in the second shell coordination in the
> Feff.inp, am I right?
>

I am not quite sure what you are getting at but I think that the answer is 
yes.  What you needto do is to have a single scattering path for each of 
the two possibilities in the second shell (Mo or W) along with the first 
shell path to O and then start fitting with minimal parameters.  For 
example, if you know that the total number of atoms in the second shell is 
12 for this structure (and you are reasonably sure that this is the 
correct structure), you can constrain the sum of the two occupation 
factors to be equal to 12 provided that you have a reasonable 
understanding of what the amplitude reduction factor might be.  You can 
also start by constraining the delta R and sigma-squareds for the two 
paths until you are sure that the fit makes sense and is stable.

If you have data from both W and Mo edges, you have more leeway in your 
fitting and you can begin to make a model which tries to figure out if 
there is clustering of one kind of atom in the structure.  There are a 
number of papers in the literature that do this (off the top of my head I 
can think of one in particular by Shibata and Bunker for Au/Ag 
nanoparticles but there are surely many more).  Ultimately you can develop 
a nice model but the one thing that you can't really do too much is 
include multiple scattering paths which include the atoms which are 
substituted for one another.

Hope this answers your question,

Carlo

-- 
Carlo U. Segre -- Professor of Physics
Associate Dean for Special Projects, Graduate College
Illinois Institute of Technology
Voice: 312.567.3498            Fax: 312.567.3494
segre at iit.edu   http://www.iit.edu/~segre   segre at debian.org