[Ifeffit] Basic questions

[Carlo Segre]

Hi Andy:

On Mon, 6 Apr 2009, Andy Connelly wrote:

>
> 1. Are there any problems if I set the R-range min to the same value as Rbkg?

No, but you shouldn't set it lower than Rbkg, in general.

>
> 2. If I want to refine the degeneracy of a path can I write the
> expression "amp*N_1" in the SO2 box? (where the N box is set to 1) and
> then refine "amp" and/or "N_1"? where N_1 equivalent to the degeneracy
> and amp is effectively SO2. Clearly they will be highly correlated,
> but does a formula like this cause IFEFFIT any problems?
>

This expression is OK, but you can only refine one or the other.  Refining 
both has no meaning.  You might as well just have a single parameter then.

> 3. If, for initial fits, I am only interested in the first shell
> should I set R-range max to a lower value to isolate that shell and
> then increase it as I look at shells at longer distances?
>

Yes, it is a good idea to start with a limited range and then move up. 
One additional positive aspect of this is that you get a better idea of 
the number of degrees of freedom you have for that shell.  If you make 
Rmax too big and you oly fit the first part of the range, you will 
artificially have too many degrees of freedom.

> 4. If I have a calculated Fe3+-O path from a 4 and 6 co-ordinate
> reference are these equivalent? Obviously, they will give a different
> bond length and the MS paths will change but if I am only fitting a
> first shell is there anything different. I think the answer involves
> the muffin tin potentials but I'm a little out of my depth there.
>

There are slight differences in the paths when you put the absorber in a 
different environment.  I have tried this in the past and I have found 
that the ultimate difference in the fit is much smaller than the estimated 
standard deviations.  The place where having the correct environment 
matters is if you are looking for second shells and there might be some 
high amplitude multiple scattering paths possible.  This is not usually 
relevant for a glass.

> Any advice about solving glass structures more than welcome. Whatever
> I try I can't get realistic results out.
>

If you send the Artemis project file, we might be able to help better.  My 
experience with glasses is that you just have to start with the minimal 
number of assumptions and try a lot of cases.  It becomes important to 
look carefully at the values of Chi that you get with different models to 
make a determination as to which one is best.  In your case, what elements 
do you have in the glass?  What do you know about the absorber's 
chemistry (i.e. does it like to be in a tetrahedral environment or does it 
prefer octahedral).  All these considerations must go into your analysis.

Carlo

> Thank you for your time,
>
> Andy Connelly
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>

-- 
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