[Wien] Bi2Se3 atomic positions in wien2k structure file.

[Martin Kroeker]

There is an entire branch of science called "Crystallography". Please do
not expect us to give the equivalent of a "crystallography 101" lecture in 
a newsgroup.
In (too) short, atomic positions as well as spacegroup assigments are determined 
experimentally, typically by X-ray diffraction. 
And while it is always possible to state which of these atomic positions 
corresponds to which Wyckoff site in the group-theoretical treatment of the 
unit cell symmetry, the reverse is usually not true. (Not all the
Wyckoff positions need to be occupied, also most have free numeric parameters
x, y, and/or z that are not determined by symmetry considerations anyway).
Wyckoff position tells you something about symmetry effects at that
point, and in cases such as your ZnAl2O4 can be used as a shorthand
notation for the coordinates - Zn on 16c, Al on 8b and O on 32e with
x=0.25. (Actually the 0.25 is a bit misleading - this is not an exact
symmetry-imposed number like the 1/8 and 1/2 of the other two sites - it 
might just as well be 0.2473 or something, just that it is the same in
x,y,z)
Normally in inorganic chemistry one would learn a few typical crystal 
structures - structure types - that certain key compound classes crystallize
in ("spinel" or "inverse spinel" in your case), and would then be able 
to generate at least an approximate structure from the knowledge that 
something crystallizes in such-and-such structure type.
For exact coordinates, consult databases and/or original publications. 
-- 
Dr. Martin Kroeker            martin at ruby.chemie.uni-freiburg.de
c/o Prof.Dr. Caroline Roehr
Institut fuer Anorganische und Analytische Chemie der Universitaet Freiburg