On Wednesday 18 April 2007, Edward L. Kunkes wrote:
Hello everyone,
I am currently using IFEFFIT to fit LIII edge data collected for Pt, Re and PtRe bi-metallic Nan particles supported on carbon, and I have several questions:
1. I get an So2 value of 0.54 from a first coordination shell fit on Re-foil (this So2 value also seems to work when fitting NH4ReO4-) Has anyone ever worked with Re before and gotten a similar value for So2 ?
That seems small. What is sigma^2?
2. I model the path for the a single coordination shell as 6 octahedrally arranged nearest neighbor atoms surrounding the absorber atom at a distance equal to that of inter -atomic spacing in the metal. When comparing the IFEFFIT predicted EXAFS for this model to one that is generated by using the first path from a crystallographic model, the two seem slightly different. The nanoparticles are very small (amorphous to XRD), and probably consist of a disordered phase, so I cannot assign a crystal structure. What should I do in this case?
There is a sizable literature on exafs and nanoparticles, including a number of excellent papers by folks whose names appear regularly on this mailing list. Reading some of those would certainly help. Your nanoparticles, like most other exafs research problems, resemble something for which you can find crystallography data, buit aren't exactly like the pure, ordered material. Because one has to start somewhere, the pure ordered material is as good a place to start as any. The trick is to figure out how your system differs from the well ordered system. It is highly likely that the first coordination shell in your nanoparticles strongly resembles the bulk material, although there might be some variability in average coordination, average interatomic disatnce, or sigma^2. The second coordination shell might deviate from the bulk a little bit more and the third a little more. Anatoly Frenkel, Scott Calvin, and Shelly Kelly all have papers on this topic.
3. I often get high correlations the coordination number and ss, or delEo and dR (the correlations are ~0.8). Using multiple k-weights doesn?t decrease the correlations. What is an acceptable value for these correlations?
Coordination and sigma^2 will usually be highly correlated -- they both affect the amplitude of chi. Similarly e0 and deltaR will usually be highly correlated because they both effect the phase. That is the nature of the exafs fitting problem. A correlation of 0.8 for either of those pairs is quite common. Without more information, it is hard to improve upon that situation. One common approach to dealing with these correlations is to do something like measure data at various temparastures. As long as your material does not change phase, you expect coordination and e0 to remain (mostly) constant with temperature which sigma^2 and delatR follow predicatable behavior. Measuring data at multiple edges is another excellent strategy when applicable. HTH, B -- Bruce Ravel ---------------------------------------------- bravel@anl.gov Molecular Environmental Science Group, Building 203, Room E-165 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007 Argonne National Laboratory phone and voice mail: (1) 630 252 5033 Argonne IL 60439, USA fax: (1) 630 252 9793 My homepage: http://cars9.uchicago.edu/~ravel EXAFS software: http://cars9.uchicago.edu/~ravel/software/