Modeling a spinel structure with fractional occupancy
Hi Matthew, Larix and its CIF Parser (based on pymatgen) can help turn such a CIF file into feff.inp files. For sites with multiple fractional occupations, the feff.inp generated will try to be randomly filled according to those fractions. Of course, with small numbers of neighboring atoms it never perfectly reflects the occupations, but it does tend to give a good guess of atoms of each species in the correct coordination sphere.
From Larix, one can go to the "Feff" Menu, then "Browse CIF Structures, Run Feff" to bring up its CIF Browser page. This will have ~20,000 CIFs from the AmMin database, but you can import most other CIF files. The one you attached imports without trouble.
Pymatgen is a little odd in assigning equivalent sites, so it identifies 6 Ni/Al sites here (4 for Ni1, 2 for Ni2). Anyway, selecting each of those generates the two attached Feff files. This CIF Browser will also run Feff for you for these files, with each run being in a different folder, and so easy to import into Larix's Feffit Page. The feff_Ni1.inp file has some Ni and Al in the Ni/Al sites at 2.85 and 3.34 Ang. Similarly, the feff_Ni2.inp file has some Ni and some Al at the Ni/Al site at 3.34 Ang.
If you want even more randomness, you could also make a tweak to the settings and re-generate feff.inp repeatedly. But since there are not many sites and the structure is pretty uniform, just having Ni1-Ni, Ni1-Al, Ni2-Ni, and Ni2-Ni and applying the fractional occupation in the fit is the simpler thing to do (and also allow testing of things like are these occupations random at a local level or is there some preferential ordering.
Cheers,
--Matt
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From: Ifeffit
participants (2)
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matthew marcus
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Matthew Newville