The structure of my material is zincblende. So normally the structure is cubic. However, when I open the primitive cell with xcrysden it dosen't look 100% cubic. It's a little bit inclined. Also when I calculated the optical properties of my material with a primitive unit cell the components of the imaginary part along x,y and z axis aren't equal (same for the real part). And normally if the structure is cubic they should be equal. Why they aren't equal?

The parameters of the conventional cell are:

  nat = 8
  ntyp = 2
  1.000000000000000   0.000000000000000   0.000000000000000 
  0.000000000000000   1.000000000000000   0.000000000000000 
  0.000000000000000   0.000000000000000   1.000000000000000 

For the primitive cell the parameters are:

  nat = 2
  ntyp = 2
  0.500000000000000   0.500000000000000   0.000000000000000 
  0.500000000000000   0.000000000000000   0.500000000000000 
  0.000000000000000   0.500000000000000   0.500000000000000 

a visualization of primitive cell using xcrysden

enter image description here

Can someone please clarify my doubts about this issue?

  • 5
    $\begingroup$ Primitive unit cell and conventional unit cells of any crystal structure are NOT necessarily the same. In your case, this is evident from the lattice parameters you provided. $\endgroup$
    – Sha
    Commented Mar 27, 2023 at 14:11
  • $\begingroup$ @Sha I know that it can be seen from the lattice parameters however the structure is cubic so for the optical components for example they should be the same along x, y and z axis, am I right? $\endgroup$
    – Camilla
    Commented Mar 27, 2023 at 14:16
  • $\begingroup$ What do you mean by the structure is cubic? Did you simulate the primitive cell or the conventional unit cell? What are your input parameters of the simulated structure? $\endgroup$
    – Sha
    Commented Mar 27, 2023 at 15:51
  • $\begingroup$ @Sha my simulated structure is primitive $\endgroup$
    – Camilla
    Commented Mar 27, 2023 at 23:12
  • $\begingroup$ Then there is nothing wrong with your simulation output, as is well elaborated by @Abdul's answer. $\endgroup$
    – Sha
    Commented Mar 28, 2023 at 0:53

1 Answer 1


I think the premise of your question is mistaken. The structure you have shown in your question is the primitive cell of Zincblende. It is not cubic. The conventional structure has a face-centered cubic (FCC) structure. But the primitive cell of an FCC structure is rhombohedral, which you got using xcrysden.

Rhombohedral crystals are also cubic but the angles are not 90 degrees. That's why you have an inclined plane. If the angle becomes 90 degree, you will have a primitive cubic structure. However, the cubic structure you are looking for is not be found here. That should be the conventional unit cell which is actually a supercell.

See this Physics SE question about the relation between the primitive and conventional unit cell of the FCC structure.

Edit: If you want to see the cubic conventional cell structure using XCrySDen, then press F4 key or select conventional cell mode from the Display menu.


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