[Ifeffit] Zinc Sulfide fitting

[Soham Mukherjee]

Dear Ifeffit Community,

Zinc Sulfide exists largely in two phases, either the cubic zinc-blende
(F-43m) phase, or the hexagonal wurtzite (P63cm) phase.

The details of the NN distances are as follows :-
1.  For cubic phase, the NN distances (Zinc to Sulfur bond lengths) are
2.3028 Angstroms each, 4 such bonds exist in a tetrahedral arrangement;
2.  For hexagonal phase, there are 2 different NN distances (Zinc to Sulfur
bond lengths) 3 bonds having the length 2.3421 Angstroms each, and 1 axially
elongated bond of length 2.3465 Angstroms. So they differ only by  0.0044
Angstroms !

I am performing a first shell fit to Zinc Sulfide which exists in the
wurtzite phase (hexagonal_P63cm). However, I could fit it quite well taking
the cubic phase (Zinc blende_F-43m) as my starting model. The only thing is
that since bond distances are higher in a hexagonal phase than the cubic
phase, I end up with a considerable delr as I underestimated my starting
bond length value. Taking a wurtzite model involves defining two different
bond lengths (which differ only by 0.0044 Angstroms !), paths 1 and 2 in the
FEFF, whose sigma^2 values are quite similar. This gives unusually large
delr values and high sigma^2 for the longer axial bond, which looks
suspicious !

Do I really need to define the two bond distances separately of so close
values? Or taking a cubic phase will be sufficient? Please find the attached
project file (artemis.apj) where I have tried out some simulations with both
cubic and hexagonal models.

Here are a few details you asked for :-

The sample is a thin layer, it's pretty homogeneous and had no pinholes
during measurements. The fit details are :-
k_min = 2, k_max = 14
R_min = 1, R_max = 3
The potential is self consistent in these calculations
Fitting was performed in the R-space

Best regards,
Soham.
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