[Ifeffit] DWF's for amorphous InP

Bruce Ravel bravel at anl.gov
Thu Mar 29 09:31:36 CDT 2007

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,

 Bruce Ravel  ---------------------------------------------- bravel at 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/

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