Dear all,
Following the previous discussion (
https://www.mail-archive.com/ifeffit@millenia.cars.aps.anl.gov/msg07118.html
) on using more than one CIF file for fitting, as Scott Calvin
and Matthew Marcus suggested, we can fold the molar ratio into
the amp parameter for each CIF. Does it mean that the overall
S02 for all phases (all used CIF files in the fitting) is "set"
to 1?
I would recommend thinking of S02 as a value that belongs to all paths of a data set, and is part of - but not the whole of - the amplitude for any path.
Typically, for the regular fitting with one model
(cif file or any simulated feff file), we take the S02 value
from the known metal foil that has been measured simultaneously
with the unknown sample.
So if we used the mentioned method and folded the
molar ratio into the amp, does it mean in Artemis we use the
following terms?
"in Artemis: S02= abs(amp) for one phase and for
another one: S02= abs(1-abs(amp)" and leave “amp” parameter to
“guess”
When mixing structures from different CIF files there can be a slightly tricky issue that each path file has a coordination number built in, and this might be different for different structures. Like, for a perfect octahedron, this "degen" would be 6, but if another structure with the same neighboring atom has a distorted octahedron, some paths might have a degen of "1" or "4" or something else.
If that is the case, I would suggest forcing all the "degen" values to 1, and then using "S02 * N1" for the amplitude term of path 1 and "S02 * N2" for the amplitude term of path 2, etc, and then define/constrain N2 to be "6- N1" (or whatever you think is appropriate total coordination is).
On the other hand, if you are mixing different phases, and you expect (for example) there is some Pt metal that is well-defined by a single structure ("CIF") and also some Pt oxide (also with a well-defined structure), and the question is "how much oxide is there", then you might leave the path degeneracies as calculated from the structures and use
"S02 * Fraction_Ptmetal" for the Pt-Pt Path from the Pt structure, and "S02 * Fraction_PtOxide" for the Pt-O Path from that structure, and then define/constrain "Fraction_PtOxide = 1 - Fraction_Ptmetal".
Either way, you can set "S02" as appropriate (say, found using a spectrum of a standard with well-known coordination", and you have easy access to trying different combinations of coordination numbers or compositional fractions.
I would appreciate having your comment on this
method or if you have any suggestion on how to deal with the S02
value in Artemis when we are using more than 2 or 3 phases (cif
files) for the fitting process.
If the idea is that you have a mixture of Pt metal, PtO, and PtCl, you could use path amplitudes of
"S02 * Fraction_Ptmetal"
"S02 * Fraction_Ptchloride"
"S02 * Fraction_Ptoxide"
let the first two fractions vary and define/constrain "Fraction_PtOxide = 1 - (Fraction_Ptmetal + Fraction_Ptchloride)".