Hey Bruce, thanks a lot for your e-mail. The correlation between the coordination numbers and the DWFs is exactly the problem. It is made worse by the overlap of the two peaks (In-P and In-In), allowing less free parameters for the fit to be physically reasonable. I was probably not specific enough with my question (apologies for that) so here some more information: Crystalline InP has a zinc blende structure with In tetrahedrally bonded to four P atoms and vice versa. However, this does not apply for the amorphous phase. The amorphous structure of compound semiconductors such as InP is characterized by structural as well as chemical disorder, i.e. the presence of wrong homopolar (In-In) bonds. The information we like to get from the EXAFS measurements (taken at the In K-edge) of the amorphous system are: (1) The total coordination number (though the total coordination number is around four, it is not yet clear whether it is exactly four, slightly over or slightly under coordinated.) (2) The percentage of the wrong (In-In) bonds. Due to the correlation between coordination number and DWF, varying ratios of the In-In and In-P DWSs give varying In-In percentages even with the total coordination number being the same. Thus we need to apply constraints (as you said) We think that restraining the DWFs might be a good approach. Unfortunately, the crystalline and amorphous systems do not have similar DWFs due to the large amount of disorder in the amorphous phase. From previous studies it is known, that the DWF of an amorphous semiconductor is roughly (!) twice the DWF of the crystalline phase. Hence, fixing the In-P DWF to twice the crystalline value (from a standard we also measured) would be a a first approach. My question is whether there maybe is a better way of relating/restraining the DWFs, as for example suggested by Crozier, Rehr and Ingalls (X-ray Absorption, Koningsberger and Prins, Wiley & Sons, 1988). They derive a formula for the DWF that contains the reduced mass and an integral over the projected density of states. If one could make a reasonable assumption about the integral (which is the problem) it would be possible to correlate the two DWFs. Or maybe there is another way to make an educated guess (by computing with FEFF or so). Hope thats a bit more clear now. Many thanks, Claudia
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/
_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
-- Claudia S. Schnohr Department of Electronic Materials Engineering Research School of Physical Sciences and Engineering The Australian National University Canberra, ACT 0200 AUSTRALIA