Very 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.
 
Anatoly
 

 

-----Original Message-----
From: Scott Calvin [mailto:scalvin@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