On Friday, March 30, 2012 12:57:21 am Ebrahim Rezaei wrote:

I am working with bulk and nano structures of MnO, MnO2 and Mn2O3. I have two questions regarding how to parameterize delr and ss in Artemis.

It is recommended to use either delr=alfa*Reff or relate it to unit cell structure. Can anybody tell me how to do the second approach?

Also is that a good idea to let all ss to float? Or it should be constrained based on type and distance of atoms? I have seen an example about ZnO just letting the first two shell’s ss to float and let the rest to be the same. I would appreciate it very much if anybody could help me with this too.

The alpha*reff trick is only strictly valid for cubic, monoatomic materials. For everything else in the world, it is an increasingly poor approximation. That's not to say it is not a helpful parameterization, just that you need to be mindful of its limitations. As for the sigma^2 parameter, you have to balance when is physically reasonable with what can be reliably extracted from a real data analysis. - Two atoms of a different species at the same distance should have different sigma^2 because they have different masses. - Two atoms of the same species at different distances should have different sigma^2 because sigma^2 is a mean square deviation in distance. So, in some sense, all scatterers might have their own sigma^2 parameters. But that is not to say that you data are sufficient to independently determine that many parameters. When you run up against the limited information content of your data, that is when you start making approximations of the sort that lead to setting sigma^2 parameters for different scatterers to be the same. However, there are sensible and physically justifiable reasons why the sigma^2 for certain multiple scattering paths should be related to those of single scattering paths. See http://dx.doi.org/10.1103/PhysRevB.54.156 Two of my favorite papers that show excellent data analysis solving real-world problems and which are written in a way that is of pedagogical value for XAS beginners are Scott Calvin's paper on doped ferrites (http://dx.doi.org/10.1103/PhysRevB.66.224405) and Shelly Kelly's paper on uranyl binding to biomass (http://dx.doi.org/10.1016/S0016-7037(02)00947-X). For more interesting papers, look through the archives of the mailing list and see who gives good answers to questions, then look up their papers. Another extremely useful resource which includes extensive discussion of the sorts of things you are asking about is Shelly's review chapter. Here's the amazon link to that book: http://www.amazon.com/Methods-Soil-Analysis-Part-Mineralogical/dp/0891188460... The book is a bit pricy, but if you are a clever googler, you may find a PDF copy of her chapter. Regards, B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 My homepage: http://xafs.org/BruceRavel EXAFS software: http://cars9.uchicago.edu/ifeffit/Demeter