Shaofeng has given me permission to repost her question here on the ifeffit mailing list. It is quoted below my response.

Dear Shaofeng,

As Bruce and I said before, a sigma2 of 0.0007 A^2 is not impossible, although it indicates less disorder than is typically present. Your attached table does seem to show some improvement by using the model from the hydrogen-containing structure as compared to the arsenate. A more rigorous test for statistical improvement can be conducted using the Hamilton test (you mentioned you’ve consulted XAFS for Everyone; full details of the Hamilton test are given there). 

It’s also encouraging that the uncertainties on your sigma2 determinations using the hydrogen-containing model are quite small; it appears that the fit is not getting confused by correlations even though it’s fitting both coordination number and sigma2, as that would generally also cause high uncertainties in the correlated parameters.

Is such a stiff sigma2 reasonable in this case? I have no idea. I just don’t know enough about this particular system; perhaps someone else on the list does. Oh, and one other question—was the data collected at room temperature? If it were collected at cryogenic temperatures, that would tend to reduce thermal disorder and thus lower sigma2’s.

Even if no one on the list has insight in to this particular system, anyone have good published examples of room-temperature systems with sigma2’s < 0.001? It might help Shaofeng with her referee…

Best,

Scott Calvin
Lehman College of the City University of New York

P. S. I certainly hope no reviewer is using the “typical values” I provide for parameters in XAFS for Everyone as rigid criteria for rejecting results! It is most certainly not the way I use them in the book. Some systems actually are atypical!

Begin forwarded message:

From: Shaofeng Wang <wangshaofeng@iae.ac.cn>
Subject: about sigma2 for exafs fitting
Date: January 4, 2017 at 8:31:33 PM CST
To: <scalvin@sarahlawrence.edu>
Reply-To: Shaofeng Wang <wangshaofeng@iae.ac.cn>

Dear Dr. Calvin,
 
I am a research from China. I know you are an expert on XAFS analysis. So I write this letter to you for some xafs analysisi problem. 
 
Recently, I am attempting to study the incorporation of arsenate into the barite structure. To investigate the species of arsenate in barite, I fitted the exafs data using Ba3(AsO4)2 and a dft optimized structure with HAsO4 incorporated in barite supercell (configuration C2), respectively and got some results. Please see the attached table. Smaller reduced chi2 and R-factor were obtained by using configuration C2 as the initial model. Does this mean the species of arsenate in barite is more likely to be HAsO42- instead of AsO43- ? We also have other evidences including XANES and vibrational spectroscopy to support this conclusion. However, using C2 as the initial model we obtained samll sigma2 (~0.0007). This seems too small and out of the normal range (0.002 每 0.03) as you mentioned in your publications (XAFS for everyone). So, my question is if our data are reasonable. If yes, could you provid some references to support?
 
By the way, I asked simialr question on the ifeffit forum and you gave me some answer. However, the reviewer was not convinced and he insited on that sigma2 must be in the range of 0.002-0.03. How can I response this question?
 
Any help is very appreciated.
 
Best regards,
 
Shaofeng
 
 
 
--------------------------------------
Shaofeng Wang, Ph.D of Geochemistry
Environmental Molecular Science Group
Institute of Applied Ecology, Chinese Academy of Sciences
Shenyang, 110016, China
wangshaofeng@iae.ac.cn
www.iae.cas.cn