Dear Julien,
A slab can never be cubic. At most it can be forced to remain
tetragonal. But I see that your perovskite contains formamidinium
cations. Even if you force the dimensions of the bulk cell to remain
cubic, the internal symmetry after relaxation will not be cubic; those
organic cations lack threefold symmetry, which is the condition for a
structure to be cubic by symmetry. This said, a correct arrangement of
the formamidinium ions might allow having tetragonal symmetry, both in
the bulk and in the slab.
José C. Conesa
El 21/02/2020 a las 13:13, Julien Barbaud escribió:
Dear users,
I am trying to simulate the surface of a perovskite. If I let vc-relax
do its job on my system, it ends up yielding a monoclinic unit cell.
However, the phase of interest for my purposes has a cubic symmetry
(actually becomes stable only at at higher temperatures). When it
comes to bulk calculations, this is no problem, because we can easily
constrain the system to a cubic symmetry and let it relax (constrained
vc-relax, or Birch fit method). And I do get very good agreement with
experimental lattice constants.
When I try to simulate a slab of a few atomic layers, however, the
vacuum thickness left in the unit cell allows for one more degree of
freedom for the slab. This results in my atomic slab "tilting" during
the relax calculation, effectively mimicking the monoclinic result
(without the actual fictitious "slab cell" changing, of course). I am
afraid this description might not be clear, so I attached an
illustration of the problem I get after a relax run on an initially
cubic slab.
My question boils down to this:*Is there a good method force a slab to
remain cubic, even though it has this additional freedom?*
This sounds like a basic question, but I couldn't find an answer so
far. I thought of limiting the relaxation the the z-axis only, but I
believe that would not be satisfying for the physics of my system
(mostly because my material is made of organic cations encaged in an
inorganic framework, and the orientation/rotation of those cations
usually are relevant. If I don't allow any movement in the x-y
directions, I won't be able to observe potential re-orientation of the
cations at the surface)
Let me know if you need any more information!
Thanks for your attention,
Julien
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--
José C. Conesa
Research Professor
Instituto de Catálisis y Petroleoquímica, CSIC
Marie Curie 2, Campus de Cantoblanco
Tel. 915854766
Madrid, Spain
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