[Ifeffit] Fe in glassy ceramics

[Scott Calvin]

Hi Andrei,

It's not unreasonable to model such a thing. Use a highly constrained  
model for the spinel, not allowing coordination numbers to float and  
using very few sigma2 parameters. I've published a model like that for  
fitting spinels here:

http://prb.aps.org/abstract/PRB/v66/i22/e224405

My model is more complicated than yours needs to be, because you don't  
really have site disorder; all the sites are Fe.

Then multiply the amplitudes by a guessed parameter representing the  
fraction of the iron that is in the spinel phase.

The remaining amplitude gets used for the glassy phase, which can  
possibly be fit by a single iron-oxygen path, which can be cloned from  
one of the spinel near neighbor paths, but now with most parameters  
floated.

You may very well need to fix S02 based on the value from a standard.

Fit over a wide range in the Fourier transform, so that you take  
advantage of high R peaks to pin down the spinel part.

Worth a shot, anyway. The key is being aggressive with assumptions and  
constraints, and then being honest about how that affects your  
uncertainties in the end.

--Scott Calvin
Faculty at Sarah Lawrence College
Currently on sabbatical at Stanford Synchrotron Radiation Laboratory


On Feb 16, 2011, at 4:53 AM, Andrei Shiryaev wrote:

> Dear colleagues,
>
> Probably similar questions were already asked, but nevertheless I  
> would appreciate some advice how to solve the problem.
>
> We are looking at Fe environment in complex glasses, which underwent  
> partial crystallization. A fraction of Fe has precipitated as spinel- 
> like nanocrystals, another fraction remains in glass. Spinel itself  
> is already challenging for XAFS, but our task is to understand the  
> Fe fraction in glass (e.g., Fe-O bond length).
>
> We do not know the exact composition of the crystalline phases, thus  
> we can not record a suitable standard. Does anybody have an idea how  
> could we try to separate the Fe in glass from Fe(cryst) contributions?
>
> I fully realize that the problem is ill-posed, perhaps, some people  
> had somehow solved this problem.
>
> Thanks a lot,
> Andrei Shiryaev
>
> Instituteof Physical Chemistry
> Moscow
> Russia