[Wien] Fluorite structure

[Michael Fischer]

Firstly, 0.25 0.25 0.25 and 0.25 0.25 0.75 are symmetrically equivalent 
positions and should only be specified once (from what you have 
written, I am not sure if you have done this correctly). The equivalent 
atoms are generated.
Secondly, I think you have a problem with your stochiometry. The 
position 0 0 0 for Al has a multiplicity of 4 atoms per unit cell, but 
0.25 0.25 0.25 means 8 nitrogen atoms per unit cell. Consequently, you 
will not arrive at AlN, but AlN2. Are you sure about the fluorite 
structure of this compound? By browsing Google for "structure AlN" and 
looking at 2 random entries, I find the keywords "wurtzite" (hexagonal 
ZnS) and "zinc-blende" (cubic ZnS), so maybe you should use one of the 
two.

Good luck,
Michael