Hi,
Most probably the proper way to do this change in Sn redox state is to
add or suppress some atom in the lattice, or change it for an ion having
naturally another valence; this is surely the way in which the mentioned
degradation proceeds. Note also that the location of the modified charge
may depend on where that modified atom position lies. Now, since the
valence band of MASnI3 is made mainly by iodine p orbitals, while the Sn
s levels lie much lower, any attempt of increasing the charge in Sn is
likely to deplete mainly the population of those iodine orbitals, not of
tin. Since the conduction band is formed mainly by Sn p orbitals it may
be easier to add electrons to Sn, for example by substituting some of
its atoms by e.g. In which will probably get a (3+) redox state.
Good luck.
José C. Conesa
El 24/04/2019 a las 6:27, JULIEN, CLAUDE, PIERRE BARBAUD escribió:
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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--
José C. Conesa
Instituto de Catálisis y Petroleoquímica, CSIC
Marie Curie 2, Madrid, Spain
www.icp.csic.es
Tel. (+34)915854766
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