[Ifeffit] How to input atoms in solid solution for Artemis

[Matt Newville]

Hi Fuxiang,

On Thu, Jun 23, 2016 at 11:38 AM, Fuxiang Zhang <zhangfx at umich.edu> wrote:

> Hi, All,
> I am new in using EXAFS techniques. Just wondering how to input atomic
> structure of NiFe solid solution to Artemis. It has a simple Ni-structure,
> but both Ni and Fe occupy the same site. If I use partial occupancy,
> Artemis program does not allow. Anyone can give me an ideal?
>

The concept of "partial occupancy" for a representation of a crystal does
not really work for modeling EXAFS.   Instead, to model EXAFS, you make a
cluster of atoms, each of which has X, Y, Z positions and a fully
determined atomic number -- an atom cannot be half Fe and half Ni.

You can start with a crystal structure of a full occupancy crystal and
create a cluster of atoms from that, then EDIT this representation of that
cluster (held in feff.inp) to build a cluster with some atoms moved,
replaced by other species, etc.    Editing feff.inp is sort of a common
answer on this list, and I'm sure that an internet search will give you
directions.   You may even find

   https://bruceravel.github.io/demeter/artug/extended/dopants.html

from the Artemis documentation helpful. ;).

Also the coordination number N cannot be refined in this simple system
> (12), is that right?
>

No, that is not right.


> In fact I just want to get the N through the fitting of EXAFS because of
> the partial occupancy.
>

One can get partial coordination numbers from EXAFS analysis.

As Anatoly points out, distinguishing "Ni-Ni" from "Ni-Fe" or "Fe-Fe" will
be extremely difficult.

Hope that helps,

--Matt
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