# Homework Help: Castep .cell file for GaN Zinc-blende

1. Dec 25, 2015

### Macalese

1. The problem statement, all variables and given/known data
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.

2. Relevant 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

3. 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.

2. Dec 30, 2015