[Ifeffit] Need an advice

Anatoly Frenkel frenkel at bnl.gov
Fri May 6 15:53:05 CDT 2005

RE: [Ifeffit] Need an adviceVery good point! And, with TEM, it is a similar
trend (I think, Scott, you mentioned it in your JAP paper): if the
distribution is broad, TEM overemphasizes larger particles because of the
resolution/contrast problems at the lower size tail of the distribution.


  -----Original Message-----
  From: Scott Calvin [mailto:scalvin at slc.edu]
  Sent: Friday, May 06, 2005 4:46 PM
  To: Frenkel, Anatoly; XAFS Analysis using Ifeffit
  Subject: RE: [Ifeffit] Need an advice

  At 04:20 PM 5/6/2005 -0400, Anatoly wrote:


  I think, the best way to deal with it is to rely on TEM or, perhaps, XRD,
to actually measure the particle size and then infer the Co-Co coordination
numbers. The Co-Co coordination numbers measured from EXAFS can then be
compared to the TEM-derived, and if the answers are different (namely, the
experimental numbers are smaller than the TEM-derived), this discrepancy can
be interpreted in terms of the "mixed phase" situation, and the mixing
fraction of metal is exactly the ratio of these two numbers. What I've just
described, is actually going to published, and I will send the reference
when it is out.

  Using TEM in this way sounds like a good idea; I look forward to seeing
this paper. But I would strongly caution people to carefully consider if
anything is known about the size distribution before using XRD to estimate
sizes. If the size distribution of crystallites is broad, XRD preferentially
"notices" the largest crystallites (it's roughly weighted as the square of
the volume). EXAFS has a different weighting (more like volume-weighting, or
even a little more biased toward smaller particles). Therefore attempts to
use an XRD-determined size to constrain EXAFS models can be highly distorted
if the size distribution is moderately broad.

  --Scott Calvin

  Sarah Lawrence College
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