[Ifeffit] Fitting EXAFS with out any atomic coordinates?

[Scott Calvin]

Hi Mario,

There are a couple of different ways to approach the kind of problem 
you describe, although you must be willing to guess at what the local 
environment looks like.

For example, maybe the chemical composition tells you that you have 
an iron oxide. At a minimum, you could use Atoms to make a crystal 
structure for, say, hematite, and then only fit the nearest-neighbor 
paths, perhaps with an unknown coordination number. This can give you 
some information.

Or, following the same example, maybe you think your material might 
be like a disordered hematite. Then you could try to fit the hematite 
structure out for several shells, but with free parameters to account 
for disorder (e.g. you could include vacancies).

The key questions that you should not lose track of:

--What would you like to know about your material?

--What do you think your material might be like?

I think sometimes people forget in particular about the first 
question. EXAFS doesn't answer generic questions very well ("tell me 
all about my sample"), but it often answers particular questions 
quite effectively ("are my iron atoms bonded to oxygen, or sulfur?"). 
So if you know what your particular questions are you've got a good 
chance of getting help using Bruce's and Matt's software.

--Scott Calvin
Sarah Lawrence College

At 02:15 PM 3/28/2007, you wrote:
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>
>Hi Bruce my name is Mario and I was just wondering if your program 
>can fir the EXAFS spectra without any atomic positions. Obviously 
>the answer is no if you use atoms to create the FEFF file but what 
>does one do when we have a semi-crystalline material for which no 
>one has attempted the fractional coordinates.
>
>I mean the XRD pattern shows some crystalline but due to the high 
>background I would think it would be very hard to do the Rietveld analysis.
>
>So what can one do, can we fit the EXAFS spectra knowing only the 
>chemical composition and perhaps the lattice geometry(orthorhombic 
>or hexagonal)?
>
>
>
>thanks
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