  This paper has been published in Bulletin of the Chemical Society of Japan,
which can be viewed at the following URL: http://doi.org/10.1246/bcsj.20150110
  Cs2SnI6, a variant of perovskite CsSnI3, is expected for a photovoltaic
material. Based on a simple ionic model, it is expected that Cs2SnI6 is
composed of Cs+, I-, and Sn4+ ions and that the band gap is primarily made of
occupied I- 5p6 valence band maximum (VBM) and unoccupied Sn4+ 5s conduction
band minimum (CBM) similar to SnO2. In this work, we performed density
functional theory (DFT) calculations and revealed that the real oxidation state
of the Sn ion in Cs2SnI6 is +2 similar to CsSnI3. The +2 oxidation state of Sn
originates from 2 ligand holes in the [SnI6]2- octahedron unit, where the
ligand [I6] cluster has the apparent [I66-L+2]4- oxidation state, because the
band gap is formed mainly by occupied I 5p VBM and unoccupied I 5p CBM. The +2
oxidation state of Sn and the band gap are originated from the intracluster
hybridization and stabilized by the strong covalent interaction between Sn and
I.
