<div dir="ltr"><div><div><div>Thank you Sir, Kroeker. for your illustration. I really appreciate you kind response.<br><br></div>Thank you once again.<br></div>Best Wishes<br></div>Naseem<br></div><div class="gmail_extra">
<br><br><div class="gmail_quote">On Fri, Feb 7, 2014 at 1:48 AM, Martin Kroeker <span dir="ltr"><<a href="mailto:martin@ruby.chemie.uni-freiburg.de" target="_blank">martin@ruby.chemie.uni-freiburg.de</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">There is an entire branch of science called "Crystallography". Please do<br>
not expect us to give the equivalent of a "crystallography 101" lecture in<br>
a newsgroup.<br>
In (too) short, atomic positions as well as spacegroup assigments are determined<br>
experimentally, typically by X-ray diffraction.<br>
And while it is always possible to state which of these atomic positions<br>
corresponds to which Wyckoff site in the group-theoretical treatment of the<br>
unit cell symmetry, the reverse is usually not true. (Not all the<br>
Wyckoff positions need to be occupied, also most have free numeric parameters<br>
x, y, and/or z that are not determined by symmetry considerations anyway).<br>
Wyckoff position tells you something about symmetry effects at that<br>
point, and in cases such as your ZnAl2O4 can be used as a shorthand<br>
notation for the coordinates - Zn on 16c, Al on 8b and O on 32e with<br>
x=0.25. (Actually the 0.25 is a bit misleading - this is not an exact<br>
symmetry-imposed number like the 1/8 and 1/2 of the other two sites - it<br>
might just as well be 0.2473 or something, just that it is the same in<br>
x,y,z)<br>
Normally in inorganic chemistry one would learn a few typical crystal<br>
structures - structure types - that certain key compound classes crystallize<br>
in ("spinel" or "inverse spinel" in your case), and would then be able<br>
to generate at least an approximate structure from the knowledge that<br>
something crystallizes in such-and-such structure type.<br>
For exact coordinates, consult databases and/or original publications.<br>
<span class="HOEnZb"><font color="#888888">--<br>
Dr. Martin Kroeker <a href="mailto:martin@ruby.chemie.uni-freiburg.de">martin@ruby.chemie.uni-freiburg.de</a><br>
c/o Prof.Dr. Caroline Roehr<br>
Institut fuer Anorganische und Analytische Chemie der Universitaet Freiburg<br>
<br>
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