On Sat, Jul 14, 2012 at 4:19 PM, johnny huertas
Hi Matt:
Thanks for your response. First of all I am beginner in this issue. Because of my little experience I don't know exactly how to specify my problem but I think you write an interesting phrase in your response: "are you asking if you can distinguish random substitution from ordered substitution? I'm not sure why the Ag-Br and Ag-Cl parameters would be the same".I don't know how to create a robust fitting model to distinguish random substitution from ordered substitution of the example found in Bruce site about AgBr(1-x)Clx. What are the parameters that I must consider to fit the spectra and to verify if exist random or ordered substitution? I am attaching a document very explicit about this. Thanks again.
Actually, I think I still don't fully understand what you are asking. When you say that the sentences I wrote are interesting, am I supposed to interpret that to mean that this is what you are asking? And, though I see the fits in your attachment, I don't really see a question that I understand. Of course, an answer to any question like "what parameters to use to fit spectra" would depend on what you're trying to model. Going out on a limb and guess that you are asking about random versus ordered substitutions, here's some thoughts that might help.... First shell EXAFS is not, by itself, sensitive to distinguishing ordered from random substitutions. That is, if there are 3 Ag-Cl and 3 Ag-Br bonds, a simple analysis of the first shell EXAFS won't tell how the Cl and Br are arranged relative to each other, only how the Br and Cl are arranged relative to the Ag. If you have a single crystal and measure the polarization dependence of the EXAFS, you might be able to distinguish the relative bonding in the different crystallographic directions. Another approach is to go past the first shell, and analyze the multiple-scattering paths. Those can be sensitive to order, as the fraction of linear Ag-Br-Ag-Cl to linear Ag-Br-Ag-Br would depend on the ordering. You might also consider measuring the Br and/or Cl edges so that, for example, you might be able to see the relative fraction of Br-Br and Br-Cl in the Br second shell. Hope that helps, --Matt