Hi Yordy: I have removed all the previous emails to keep the message short... On Sat, 22 Nov 2008, Yordy Licea Fonseca wrote:
Hi Carlo and Ying,
Does it means that if the sample is a random solid solution at experimental temperature, in principle, you would be able to simulate the EXAFS spectra whichever be the Mo/W ratio in the second shell coordination in the Feff.inp, am I right?
I am not quite sure what you are getting at but I think that the answer is yes. What you needto do is to have a single scattering path for each of the two possibilities in the second shell (Mo or W) along with the first shell path to O and then start fitting with minimal parameters. For example, if you know that the total number of atoms in the second shell is 12 for this structure (and you are reasonably sure that this is the correct structure), you can constrain the sum of the two occupation factors to be equal to 12 provided that you have a reasonable understanding of what the amplitude reduction factor might be. You can also start by constraining the delta R and sigma-squareds for the two paths until you are sure that the fit makes sense and is stable. If you have data from both W and Mo edges, you have more leeway in your fitting and you can begin to make a model which tries to figure out if there is clustering of one kind of atom in the structure. There are a number of papers in the literature that do this (off the top of my head I can think of one in particular by Shibata and Bunker for Au/Ag nanoparticles but there are surely many more). Ultimately you can develop a nice model but the one thing that you can't really do too much is include multiple scattering paths which include the atoms which are substituted for one another. Hope this answers your question, Carlo -- Carlo U. Segre -- Professor of Physics Associate Dean for Special Projects, Graduate College Illinois Institute of Technology Voice: 312.567.3498 Fax: 312.567.3494 segre@iit.edu http://www.iit.edu/~segre segre@debian.org