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

[Marzari Nicola]

Here is a discussion http://theossrv1.epfl.ch/Main/OxidationStates

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On 24 Apr 2019, at 00:28, JULIEN, CLAUDE, PIERRE BARBAUD <julien_barbaud at sjtu.edu.cn<mailto:julien_barbaud at sjtu.edu.cn>> wrote:

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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