On Thursday 29 March 2007 02:57, Claudia Schnohr wrote:
Hello everyone.
I am a PhD student and I have encountered a problem with analysing the EXAFS of amorphous InP.
For amorphous InP the first shell around an In atom is comprised of both P and In atoms. The In leads to a small peak in the R-spectrum that strongly overlaps with the bigger peak due to scattering from P. If I use two different Debye-Waller-factors, one for each scatterer, and let them both float during the fit I get weird values since the coordination numbers for both peaks have to be floated as well. Therefore, some restraint is needed for the DWF's.
Is there any correlation between the two DWF's following from theory or experiment that I could use to restrain my fitting parameters ? Are there other possibilities to handle such a situation ?
Many thanks in advance for your help,
Hi Claudia, If I understand your explanation, I suspect that the problem is that your fit has more freedom in its parameters than the data can support. It is always the case that coordination number and sigma^2 are highly correlated. They are both terms that affect the amplitude of chi. I doubt that the solution is somehow to constrain the sigma^2 values. Without doing some serious theory to figure out how those two values might be related, I would not know what constraint to apply. What would be a lot more reasonable would be to constrain the total number of atoms in the coordination shell. I don't know what kind of crystal InP forms, but I would assume that the In is either 4- or 6-coordinated with P in the crystal. It seems reasonable to enforce that coordination in the amorphous material. That is, require that the sum of In and P atoms in the first coordination shell be 4 (or 6 or whatever). Make a guess parameter that describes the amount of the In: set n = 4 # (or 6 or whatever) guess x_in = 0.1 def x_p = n - x_in then define you sigma^2 parameters as before: guess ss_in = 0.003 guess ss_p = 0.003 That reduces the number of parameters in the fit by one, enforces a physically reasonable constraint on the total number of parameters, and -- hopefully -- helps to stabilize your fit by removing one of the highly correlated guess parameters. As I re-read what I wrote, it occurs to me that another reasonable constraint might be to require that sigma^2 for the In-P bond be the same in the amorphous material as in the crystal. Did you measure crystalline InP as well? Hope that helps, 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/exafs/