[QE-users] forcing cubic symmetry for a slab

[José Carlos Conesa Cegarra]

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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