[Ifeffit] atoms and the alloys

[Scott Calvin]

Hi Mauro,

Nope; you're using Atoms right. Atoms properly only simulates regular 
crystals (i.e. something with a space group), and a random alloy 
doesn't quite qualify. (If there was a regular pattern to the 
substitution, then there would be a superstructure, and you wouldn't 
need to give shared occupancy.)

That doesn't mean, however, that we haven't found all sorts of ways 
of using Atoms to do non-crystallographic things. But yes, you do 
have to modify the feff.inp by hand to handle an alloy. I describe 
this procedure at:

http://cars9.uchicago.edu/iffwiki/Doped?highlight=%28CategoryNotBulkCrystals%29

I will point out that in your case you actually don't have to worry 
too much; all you should have to midfy is the central (absorbing) 
atom. Why? Because copper and nickel scatter electrons in a very 
similar way, and EXAFS is generally not considered to be sensitive to 
the differences. Thus it's OK to model your alloy as all fcc Ni with 
a copper at the absorbing site for the Cu edge, and just plain fcc Ni 
for the Ni edge. Note that bond lengths, ss's, and E0's may differ, 
but you're presumably fitting those anyway...the feff paths 
themselves should be fine.

--Scott Calvin
Sarah Lawrence College

>Hi to all,
>
>I have noticed that Atoms fails to generate a correct "feff.inp" file
>in the case of alloys. For example, let's take the Ni_xCu_{1-x},
>x=0.5, an fcc substitutional. If I put Ni as central atom and give 0.5
>occupancy on the shared Cu-Ni site (0,0,0.5) the result is a normal Ni
>fcc without Cu.
>
>At the moment I modify my feff.inp by hand to simulate alloys, but
>maybe I'm just not using Atoms properly. If this is the case, it could
>be useful to insert your answer in the FAQ.
>