Atom position (coordinates) from symmetry

[pie86]

Hi guys,
I need to write down the positions of all the atom in LaFeAsO unit cell:

Figure caption: "The quaternary equiatomic ZrCuSiAs-type structure is very simple, with only eight atoms in the tetragonal cell. The dashed lines represent a unit cell."

I also have these informations about ZrCuSiAs

Atom Posit x y z

Zr 2c 1/4 1/4 0.2246
Cu 2b 1/4 3/4 1/2
Si 2a 1/4 3/4 0
As 2c 1/4 1/4 0.6793

Space group: P4/nmm.

So...starting from the figure above is easy to get the coordinates of Fe and O atoms but what about Sm and As?
And what's the relationship between the image and the positions reported above (and note that the pictures reported in the article about ZrCuSiAs and the above one from nature are different! http://img541.imageshack.us/img541/3563/zrcusias.jpg" )

And last but not least...is there a tool to automate this process? Writing down all x,y,z is really boring!:)

Thanks
Pietro

 

[carguies]

Ok, the two upper green balls are in the left and right faces, then b=0 and b=1. Both are at half face, a=1/2. And height c is 1/2 plus "x" were x you should find aplying trigonometrics. Maybe d Fe-As = SQROOT(2)*x
The lower green balls are almost the same a=1 a=0, b=1/2 c=(1/2)-x

And for the blue ones the same.

 

[Rajini]

Hi,
Have a look at this site
http://cst-www.nrl.navy.mil/lattice/struk/FeS2.html

There are are some coordinates for writing x,y, and z values.
e.g., few are: cartesian, internal coordinates, and z-matrics.

May for visualization you can use MOLDEN or Avogadro (both are free)