[QE-users] forcing Sn(iv) or Sn(0) charges in a MASnI3 crystal

[dv009200 at fh-muenster.de]

Dear Julien,

I remember that some time ago as I learned about Quantum ESPRESSO I viewed
an old conference contribution (from the Quantum ESPRESSO Tutorial
recommendations) about DFT+U from Matteo Cococcioni. There he had an
example where the calculation of Li0.5FePO4 led to an charge of 2.5 on the
Iron atoms with normal DFT. With DFT+U he ended up with half of the Iron
atoms in the oxidation state +2 and the other half of the Iron atoms in
the oxidation state +3 which is physically more reasonable than the normal
DFT result. I don't know how well this applies to your problem but if you
want to check the conference contribution here's the link:
http://slideshot.epfl.ch/play/kQtOP4iZGwFA

Hopefully this can help

Best regards

Dominik


M.Sc. Dominik Voigt PhD Student
Münster University of Applied Sciences
Email: dv009200 at fh-muenster.de

> Dear users,
>
>
>
> I am working on a MASnI3 crystal. In this structure, the Sn can usually be
> considered as a Sn2+ cation. I ran some calculations on the system, and
> performed a Bader charge analysis on an all-electron paw charge density.
> It
> seems to confirm that the tin is in Sn(2) configuration with bader charge
> 48.3 instead of 50 (this is, by the way, inconsistent with the results of
> Lowdin analysis as implemented in projwfc.x, which gives pretty much the
> full 4d10 5s2 5p2 orbitals of Sn(0) ).
>
>
>
> So everything is as expected so far (from the Bader point of view at
> least).
> However, I would like to model a MASnI3 with a "defect" consisting of an
> "oxidized or reduced" tin atom given by Sn(iv) or Sn(0) in this material.
> Indeed, it was reported in J. Mater. Chem. A, 2018,6, 21389-21395 that
> some
> degradation mechanisms can lead to the presence of such states, and I want
> to explore their consequence on the material properties.
>
>
>
> However, I am not sure how to tackle this. My first idea was that I
> probably
> needed to create a pseudopotential with 2 missing or additional valence
> electrons. But on second thought, this method might be valid if we have
> missing core electrons, but not for valence. I highly doubt that  it would
> give me the expected result once I place it in the crystal lattice, given
> that there is really no reason for those additional electrons to gently
> "stay on their starting atom", so to speak.
>
>
>
> So is there a reliable method to study such a system by "forcing" an
> oxidization state for an atom in a crystal ? This task seems to be made
> difficult by the very subjective definition of an "atomic charge" in the
> framework of quantum mechanics and DFT.
>
>
>
> Thanks in advance
>
> Julien
>
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