[Ifeffit] Basic questions

Andy Connelly andyjconnelly at googlemail.com
Mon Apr 6 12:15:47 CDT 2009

Some really quick basic questions. I am working with EXAFS spectra of
silicate glasses at various edges (including S and Fe K edges) and I
am having a really difficult time pinning down the structures. Clearly
with glasses I can't always be sure of the oxygen co-ordination and so
I have to refine this value which is one among many problems. Just to
check I have things right:

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

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?

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?

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.

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

Thank you for your time,

Andy Connelly

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