[Ifeffit] Hexagonal WO3

[Carlo Segre]

Hi Ying:

On Tue, 18 Nov 2008, Ying Zhou, Anorganisch-Chemisches Inst. wrote:

> Dear All
>
> Recently, I got one compound, W/Mo oxide, which has the similar structure
> with hexagonal WO3. We tested the EXAFS of W L3 edge and Mo K edge of this
> compound. So, we use the hexagonal WO3 model to refine the structure of our
> compounds. My question is that how can I get the FEFF file of this compound.
> Can I directly change some of W to Mo in FEFF file? or Do I need to add the
> Mo in the atom file? I do appreciate your help.
>

There are several ways to do this, depending on what you are trying to 
find out.  If you change W to Mo in the atoms input file than you are 
really making MoO3.  What it seems like you need to do is to generate the 
FEFF input file from the WO3 structure, then replace the central atom with 
Mo.  This will give you the first path which is likely oxygen.  The second 
longest set of paths are likely to be W/Mo and you can do two things, one 
is to generate a FEFF calculation with W in this position and then make a 
second calculation with Mo and use both paths to fit the Mo environment. 
You will have to be careful about multiplicities though since by starting 
with the WO3 structure, you are assuming some degeneracies in the paths. 
The advantage is that you potentially have some multiple scatteing paths 
available to use.

Another approach is to generate the FEFF input file from WO3, look at the 
near neighbors and then generate some "quick first shell" paths for the 
nearest neighbor (oxygen) and then two fro the next near neighbor (one 
with W and one with Mo).  The difference here is that you now have 
uncorrelated paths with no implied degeneracies.  You cannot use this to 
fit multiple scattering paths.

I guess that my approach would be to start with the latter, more simple 
approach first to try to understand the problem.  If I see that the 
environment looks like it is possible to fit with multiple scattering, I 
would move to the first approach, being careful to realize that in a solid 
sloution compound, the occupancy of the second shell of metal atoms is 
probably random and not perfectly ordered like a FEFF input file with 
substitutions would be.

Hope this helps.

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