Dear Dr. Mostafa Youssef,

 Thank you very much for your kind reply. I am trying to build a supercell.
I got another reply in this forum that there has to be at least two Nb
atoms as there is no Wyckofff positions having only 1 atom in the space
group 194. If I want to use the Wyckoff positions I have to use the ones
having no x, y, z parameters in them, because at present I have no
crystallographic information about the Zr-Nb alloy. I may assume that Zr
atoms are in the Wyckoff positions, 2b, 2c, 2d and 6g positions (=total 12
Zr atoms) and Nb atoms are in 2a positions (=2 atoms). These are also
assumptions but at present I do not have any other options to start with.
Of course I suppose this will keep the space group intact, i.e., if I put
the atoms in these Wyckoff positions quantum espresso will be able to
determine the space group as 194.

 Please let me know your comments and suggestions regarding building up the
supercell.
Thanking you,
Yours sincerely,
Krishnendu
-------------------------------------------------------------------------------------------------------------------------------

You wrote:

Dear Krishnendu,

If you model the alloy using a unit cell, then you are modeling a very high
Nb concentration ordered alloy.

Take HCP unit cell having two Zr atoms. If you replace one with Nb, then
you get 50%-50% Zr-Nb.

There is no way to avoid forming a supercell of Zr (Say 54 atoms) and then
replace one with Nb.


Regards,
AUC, New Cairo, Egypt

-- 
Dr. Krishnendu Mukherjee,

Principal Scientist,
CSIR-NML,
Jamshedpur.
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