[Pw_forum] Fwd: beta tin structure

[Mike Mehl]

All right, the fourth time is a charm:

The full tetragonal unit cell of beta-tin has four atoms in it.

However, the primitive body-centered unit cell as two atoms in it.

You may want to look up the beta-tin structure, e.g.

http://cst-www.nrl.navy.mil/lattice/struk/a5.html

and see how you can construct the full tetragonal cell with four atoms 
from the primitive body-centered tetragonal unit cell containing two atoms.

Understand this and you'll understand the origin of your 6/12 problem.

On 01/04/2012 03:48 AM, bhabya sahoo wrote:

> i want the phonon dispersion curve for beta tin structure
> so i gave the positions of atoms as primitive tetragonal structure
> having coordinates
> 0 0 0, .5 0 .25 , .5 .5 .5, 0 .5 .75 like 4a whychoff positions (c/a=.546)
> so no of atoms is 4 in the unit cell so in phonon there will be 12 branches
> but i have seen 6 branches so how can i put my primitive tetragonal
> structure having two atoms in unitcell
> or in other way how can i give the input for beta tin structure

-- 
Michael J. Mehl
Head, Center for Computational Materials Science
Naval Research Laboratory Code 6390
Washington DC