[Ifeffit] atoms and the alloys

[Scott Calvin]

Hi Norbert,

I'm comparing the answers we just gave. I have almost never used FEFF 
for XANES...would you agree that =EXAFS= is insensitive to the 
difference between Ni and Cu scatterers (except, of course, for the 
structural differences that can be handled by fitting parameters such 
as delr, ss, and e0)?

--Scott Calvin
Sarah Lawrence College

>Dear Mauro,
>
>FEFF is very sensitive towards subtle changes in the atom list as caused by
>introducing a vacancy or replacing a Ni by a Cu atom as you describe. Imagine
>an FCC system with 50% atom A and 50% atom B. ATOMS has no a priori knowledge
>how it should distribute A and B in the different shells. 
>Unfortunately each of
>the possible combination will give you a slightly (or heavily) different XANES
>signature.
>ATOMS could use a random number generator to decide how a certain 
>point in space
>should be occupied. In the above example it would give each atomic position in
>the AB alloy a 50% chance to be either A or B. This will result in a 
>FEFF input
>file corresponding to only one of many possible atomic permutations. This
>particular atomic configuration might be close to reality - but it might as
>well be unreasonable.