I would make a feff.inp file corresponding to the Co as a central atom in CoGa 
in its original CsCl structure. Then, I would substitute all Ga atoms by Co 
(the two sites are crystallographically equivalent in the bcc lattice, and 
there is no need to do it separately for Cs and Cl sites). That would give me 
Co-Co contributions for all single scattering, and Co-Co-Co...-Co contributions 
for all multiple-scattering paths. 
Then, I would do the same by making a feff.inp file for Ga as an absorber in 
its CsCl structure. Then, again, substitute all Co for Ga and create Ga-Ga and 
Ga-Ga-Ga...--Ga contributions.
 
If you REALLY want to include multiple-scattering contributions of collenear 
paths (going from the corner through the body center through the opposite 
corner along the cube diagonal) you would have to also generate mixed 
multiple-scattering paths by leaving one and/or two shells around Co(Ga) 
un-substituted in such models. That would give you Co-Ga-Ga, Co-Ga-Co, etc. 
linkages and their corresponding double and triple scattering collinear paths.
In short, it is just to make a number of models and save the resulting 
feffxxxx.dat paths on your disk for each of them, and make a table for 
yourself, which one corresponds to what linkage.
 
Then, you can add them all together in your fitting model and vary their mixing 
fractions. For example, if you want to fit the first shell only, you should fit 
it as: x*chi(Co-Co) + (1-x) chi(Co-Ga) for Co edge. It is of course best to do 
it by fitting Co and Ga edge concurrently, within the same run of your fitting 
program, by imposing multiple constratints on your Co-Ga distances, mixing 
fractions and debye-waller factors.
 
There are large number of papers describing multiple data set (multiple edge) 
fitting schemes in alloys which use this very simple method.
 
Anatoly 
 
 


________________________________

From: ifeffit-boun...@millenia.cars.aps.anl.gov on behalf of Shashwat
Sent: Fri 11/6/2009 9:37 PM
To: ifeffit@millenia.cars.aps.anl.gov
Subject: [Ifeffit] modeling anti-site disorder


Dear All,
 
I am studying the local disorder caused in CoGa alloy (B2: CsCl structure) due 
to ball-milling; it is expected that Ga atoms substitute most of the Co atoms 
in the Co sublattice after prolonged milling. In order to model this 
"anti-site" disorder (occupation of Ga atoms on the Co sites) in ATOMS, I am 
considering to change the occupancies at the corresponding sites as given below:
 
Before ball-milling:
 
site a: occupancy of Ga = 1
site b: occupancy of Co = 1
 
(standard .cif data)
 
After ball-milling (assuming 80% anti-site concentration):
 
site a: occupancy of Ga = 0.2 (plus compensating vacancies)
site b: occupancy of Co = 0.2
site b: occupancy of Ga = 0.8
 
Could anyone suggest if this approach is reasonable. 
 
Many thanks ...
 
Regards,
 
Shashwat
 
 

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