Hi Andy: On Mon, 6 Apr 2009, Andy Connelly wrote:
1. Are there any problems if I set the R-range min to the same value as Rbkg?
No, but you shouldn't set it lower than Rbkg, in general.
2. If I want to refine the degeneracy of a path can I write the expression "amp*N_1" in the SO2 box? (where the N box is set to 1) and then refine "amp" and/or "N_1"? where N_1 equivalent to the degeneracy and amp is effectively SO2. Clearly they will be highly correlated, but does a formula like this cause IFEFFIT any problems?
This expression is OK, but you can only refine one or the other. Refining both has no meaning. You might as well just have a single parameter then.
3. If, for initial fits, I am only interested in the first shell should I set R-range max to a lower value to isolate that shell and then increase it as I look at shells at longer distances?
Yes, it is a good idea to start with a limited range and then move up. One additional positive aspect of this is that you get a better idea of the number of degrees of freedom you have for that shell. If you make Rmax too big and you oly fit the first part of the range, you will artificially have too many degrees of freedom.
4. If I have a calculated Fe3+-O path from a 4 and 6 co-ordinate reference are these equivalent? Obviously, they will give a different bond length and the MS paths will change but if I am only fitting a first shell is there anything different. I think the answer involves the muffin tin potentials but I'm a little out of my depth there.
There are slight differences in the paths when you put the absorber in a different environment. I have tried this in the past and I have found that the ultimate difference in the fit is much smaller than the estimated standard deviations. The place where having the correct environment matters is if you are looking for second shells and there might be some high amplitude multiple scattering paths possible. This is not usually relevant for a glass.
Any advice about solving glass structures more than welcome. Whatever I try I can't get realistic results out.
If you send the Artemis project file, we might be able to help better. My experience with glasses is that you just have to start with the minimal number of assumptions and try a lot of cases. It becomes important to look carefully at the values of Chi that you get with different models to make a determination as to which one is best. In your case, what elements do you have in the glass? What do you know about the absorber's chemistry (i.e. does it like to be in a tetrahedral environment or does it prefer octahedral). All these considerations must go into your analysis. Carlo
Thank you for your time,
Andy Connelly _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
-- 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