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
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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?