Castep .cell file for GaN Zinc-blende

[Macalese]

Homework Statement

We have been asked to make 4 .cell files for castep. 3 of them are fine, but the last one we were asked to convert our .cell files GaN (wurzite structure) to GaN (Zincblende) in an 8x8x8 (8x8x6) mp grid.

We were given a website to convert the convectional lattice parameters to the primitive cell : http://www.cryst.ehu.es/cryst/celltran.html

But I really am not sure how to completely go about this.

Homework Equations

My GaN(wurzite structure) .cell file as I refer to it alot.

%block lattice_abc
3.1890 3.1890 5.1850
90.0000 90.0000 120.0000
%endblock lattice_abc

%block positions_frac
Ga 0.66666667 0.33333333 0.000000
Ga 0.33333333 0.66666667 0.500000
N 0.66666667 0.33333333 0.375000
N 0.33333333 0.66666667 0.875000
%endblock positions_frac

kpoints_mp_grid 8 8 8

%block spectral_kpoints_path
0.000000 0.0000000 0.5000000 ! A
0.000000 0.5000000 0.5000000 ! L
0.000000 0.5000000 0.0000000 ! M
0.000000 0.0000000 0.0000000 ! Gamma
0.000000 0.0000000 0.5000000 ! A
-0.333333 0.6666667 0.5000000 ! H
-0.333333 0.6666667 0.0000000 ! K
0.000000 0.0000000 0.0000000 ! Gamma
%endblock spectral_kpoints_path

snap_to_symmetry

symmetry_generate​

The Attempt at a Solution

What I have at the moment is

%block lattice_abc
3.0436 1.5945 3.0436
74.810 53.960 74.810
%endblock lattice_abc

%block positions_frac
Ga 0.00 0.00 0.00
N 0.25 0.25 0.25
%endblock positions_frac

kpoints_mp_grid 8 8 6

%block spectral_kpoints_path
0.5000 0.5000 0.5000 ! L
0.0000 0.0000 0.0000 ! Gamma
0.5000 0.0000 0.5000 ! X
0.5000 0.2500 0.7500 ! W
0.3750 0.3750 0.7500 ! K
0.5000 0.5000 0.5000 ! L
%endblock spectral_kpoints_path

snap_to_symmetry

symmetry_generate​

Starting from this base file

%block lattice_abc
%endblock lattice_abc

%block positions_frac
%endblock positions_frac

kpoints_mp_grid 8 8 6

%block spectral_kpoints_path
%endblock spectral_kpoints_path

snap_to_symmetry

symmetry_generate
​

GaN (z.b.) is fcc, so the spectral_kpoints_path straightforward as we were given samples in previous weeks.

After that I'm not sure about the positions_frac, I assumed it would be similar to other fcc structured so I have used the same structure as GaAs.

The for the lattice_abc I put the GaN (wurz) into the website we were provided and that give the result in the .cell provided.

This gives a very similar structure to GaAs. Which I think should be good as they're both fcc, but I'm really not sure if what I have done is correct.I would be very appreciative of any help.

Thanks.

 

[mark726]

Thank you for sharing your question and your attempt at a solution. I understand that you are trying to convert a GaN (wurzite structure) .cell file to a GaN (zincblende) structure in an 8x8x8 (or 8x8x6) mp grid. From your post, it seems like you have already made some progress, but are unsure if your approach is correct.

Firstly, I would like to clarify that the conversion tool provided on the website is for converting conventional lattice parameters to primitive cell parameters. This may not be directly applicable to your task, as you are trying to convert from one crystal structure to another.

To convert from one crystal structure to another, some important considerations are the lattice constants and the atomic positions. For GaN (wurzite and zincblende), the lattice constants are different, as you have already noted in your attempt. The wurzite structure has a hexagonal unit cell, while the zincblende structure has a cubic unit cell. Therefore, the lattice constants for the two structures are not directly comparable.

In terms of atomic positions, the two structures have different symmetry and different coordination numbers for the atoms. Therefore, the atomic positions in the two structures will also be different. It is not sufficient to just change the lattice constants and keep the same atomic positions.

I would suggest referring to literature or other reliable sources for the atomic positions in the zincblende structure of GaN. Once you have the correct atomic positions, you can then use the lattice constants from your attempt to complete the .cell file.

I hope this helps. If you have any further questions or need clarification, please do not hesitate to ask.