My Community

Consider a simple perovskite, ABO3. Is VASP capable of keeping the B and O-site ions fixed, while allowing the A-site ions to relax into an optimum geometry?

Thanks for any responses in advance.

I think the answer is no.

By symmetry definition this makes no sense.

POSCAR...
<span class='smallblacktext'>[ Edited Wed Jul 06 2011, 08:03PM ]</span>

I was told that within the POSCAR file I can define which ions are fixed and with are not during my relaxation. I refer you to the VASP manual where the POSCAR file is described, it states "The seventh line switches to 'Selective dynamics' (only the first character is relevant and must be 'S' or 's'). This mode allows to provide extra flags for each atom signaling whether the respective coordinate(s) of this atom will be allowed to change during the ionic relaxation. "

The passage below is also taken from the VASP manual:

Cubic BN
3.57
0.0 0.5 0.5
0.5 0.0 0.5
0.5 0.5 0.0
1 1
Selective dynamics
Cartesian
0.00 0.00 0.00 T T F
0.25 0.25 0.25 F F F

Notice the red "F"'s theses are the tags necessary for fixing just one axis or all three.

And I do hope that the symmetry of the system will be lowered. Do you still think that this approach is nonsensical?

And thanks for the dialogue!!

<span class='smallblacktext'>[ Edited Wed Jul 06 2011, 08:05PM ]</span>

Ultimately my goal is to incrementally tilt the octahedra within the ab-plane, then relax the A-site cation.

With the POSCAR above this is nonsense. You would have to move to a (larger) supercell if you want to study this effects you described. Then you rotate a bit and you succeed with your kind of relaxation ... (But it'll become symmetric again ...)

Cheers,

Alex

Ah,
The POSCAR above is nonsense to me as well! It isn't my system, I wish to study a perovskite, ABO3, and I do not want everyone to know the specifics =). I was just hoping to show that it is possible to keep certain ions fixed while allowing others to relax via the POSCAR "setup". As I stated above, I took the POSCAR above straight from the VASP manual.

But back to the symmetry argument....
(But it'll become symmetric again ...)
What do you mean by that, could you please elaborate.


<span class='smallblacktext'>[ Edited Thu Jul 07 2011, 05:26PM ]</span>

For ABO3 the spinel structure should be more stable than a distorted spinel. So it will relax back.
You may introduce impurities (e.g. other atoms) to somehow fix a distortion.
You may also try on your own and yuo will see and learn.

Cheers,

alex

I thought that the AB2O4 is the spinel structure. Ideally i'd like to lower the symmetry of perovskite ABO3 from Pm3m to Pbnm. And your right I will definitely play with the system to see how it behaves. Thanks!

yep, my mistake. sorry.