[Ifeffit] Crytallites AND polycrystalline - Artemis handling?

[Scott Calvin]

Sure, Joseph.

These were nanoparticles made with a reverse micellar method. Past  
experience has shown that this synthesis produces a passivating oxide  
layer on an iron core; the passivating layer is usually amorphous,  
which is a good thing; hematite has a lattice mismatch and thus  
doesn't passivate. Linear combination analysis suggested this samples  
was 75% metal and 25 +/- 5% oxide.

A typical amp entry is:

S02_Fe*Frac_bcc*(1-3/4*reff/R_Fe+1/16*(reff/R_Fe)^3)

S02_Fe is just the usual S02 factor, and "should" be from 0.70 to 1.0.  
It's lower than that in the fit I've sent you; that's one of the  
indications the fit may not yet be done. (I've actually gone much  
further with this fit, but that would have made for a more confusing  
file.)

Frac_bcc is the fraction of iron atoms in an ordinary bcc metal  
environment. In the fit I sent you, it optimizes to 34 +/ 9%. Notice  
that's consistent with the linear combination analysis. That makes me  
happy.

R_Fe is the radius of the core, modeled as a sphere. The formula in  
which it appears accounts for the reduction in average coordination  
number due to surface termination, and is discussed in several of my  
papers, including my articles:

S. Calvin et al., Appl. Phys. Lett. 87, 233102 (2005)

and

S Calvin et al, Phys. Scr. T115 744-748 (2005).

The oxide amp entry is much simpler:

S02_Fe*Frac_Ox*2

Frac_Ox is 1-Frac_Bcc; i.e. what's not metal is oxide.

The factor of 2 is just because I was working with a path from  
hematite that had coordination number of 3 (there's some splitting in  
hematite); I multipled by 2 to get an assumed coordination number of  
6. Much of my subsequent fitting has been using bond valence theory to  
try to distinguish between coordination of 4 or 6 for the amorphous  
oxide.

--Scott Calvin
Sarah Lawrence College

On Oct 20, 2008, at 8:46 AM, Joseph Washington wrote:

> Scott,
> Thank you very much. I am looking at this file and I understand  
> everything as far as the isotropic expansion terms in the delr and  
> the debye term. Can you clarify the amp math expression? Also, just  
> so we're on the same page, tell me a little about this system?
>
> Thank you again for your help!
>
> Joseph
>
> Scott Calvin wrote:
>> Hi Joe,
>> Here's an in-progress Artemis file for a system with some broad  
>> similarities to the one your describe. Perhaps it will help.
>> Disclaimer: This is /not/ a final fit!
>> --Scott
>> ------------------------------------------------------------------------
>> On Oct 17, 2008, at 2:15 PM, Joseph Washington wrote:
>>> Hello all,
>>> I am working on analyzing Ge edge EXAFS data of an annealed sample  
>>> which
>>> is likely to be Ge-Te crystallites with some amorphous material at  
>>> the
>>> grain boundary (or perhaps, just Ge-N crystal material at the
>>> interface). I do not have any information about the grain size, only
>>> possible constituents for the grain and the boundary. Can anyone  
>>> give me
>>> some examples of how Artemis might handle this?
>>>
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