Changing the orientation of the unit cell

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Hola
Posts: 9
Joined: Thu Mar 29, 2018 10:51 am

Changing the orientation of the unit cell

Post by Hola » Mon Jul 22, 2019 8:20 pm

Dear all,

I am studying bulk materials, a FCC and a BCC one. For both, I considered
a conventional unit cell
rprim
1 0 0
0 1 0
0 0 1

For the FCC, I have 4 atoms with
xred
0.0 0.0 0.0
0.5 0.5 0.0
0.0 0.5 0.5
0.5 0.0 0.5

and for the BCC I have 2 atoms with
xred
0.0 0.0 0.0
0.5 0.5 0.5

I want to change the orientation of my unit cell. In fact, I would like to consider
the (100), (110) and (111) surface orientations. The idea is to make a bulk uniaxial
relaxation along these three directions, and so to increase the distance between
theses atomic planes.

For each surface orientation, how should I change rprim and xred, to allow a
relaxation of the bulk in a direction perpendicular to the surface ?

Best regards.

ebousquet
Posts: 469
Joined: Tue Apr 19, 2011 11:13 am
Location: University of Liege, Belgium

Re: Changing the orientation of the unit cell

Post by ebousquet » Tue Jul 23, 2019 10:57 am

Dear Hola,
There are some crystallographic programs that you can use to build your surfaces, you can handle that with VESTA for example.
Best wishes,
Eric

Hola
Posts: 9
Joined: Thu Mar 29, 2018 10:51 am

Re: Changing the orientation of the unit cell

Post by Hola » Thu Aug 29, 2019 12:56 am

Dear Eric,

Thank you for your response. In the Abinit tutorial n°4, to determine the (100) surface
energy of the FCC Aluminium, they first change the orientation of the cell by defining a supercell with new primitive vectors :

rprim
0.5 -0.5 0.0 # These values will define a cell with two atoms,
0.5 0.5 0.0 # non-primitive, with a different orientation
0.0 0.0 1.0 # than the primitive cell.

with xred
0.0 0.0 0.0
0.5 0.5 0.5

and then they increase the length of the third vector :
rprim
0.5 -0.5 0.0
0.5 0.5 0.0
0.0 0.0 2.0

I am not interested by surface energy, but by bulk uniaxial relaxation. I was wondering, If I define the FCC Aluminium with conventional vectors :
acell 3*7.5
rprim
1 0 0
0 1 0
0 0 1
xred
0.0 0.0 0.0
0.5 0.5 0.0
0.0 0.5 0.5
0.5 0.0 0.5

and if I increase the lenght of the third vector with acell becoming :
acell 2*7.5 9

in this way, do I make a relaxation of the bulk along the (001) direction ?

Best regards.

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