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.
To avoid possible electroweak vacuum instability in the vector-like fermion model, we introduce a new singlet scalar to the model, which couples to the vector-like fermion, and also mixes with the Higgs boson after spontaneous symmetry breaking. We i
nvestigate the vector-like fermion predominantly coupled to the third generation quarks, and its mass is generated from the vacuum expectation value of the new scalar field in the model. In this setup, as running towards high energies, the new scalar provides positive contribution to the running of the higgs quartic coupling, and the matching on the scale of the scalar mass gives rise to a threshod effect that lifts up the higgs quartic coupling strength. The two effects help stabilize the electroweak vacuum of the Higgs potential. Therefore, this setup could evade possible vacuum instability in the vector-like fermion model. We show that a large range of parameter space is allowed to have both stable Higgs vacuum and perturbativity of all the running couplings, up to the Planck scale. We also examine the experimental constraints from the electroweak precision observables such as oblique corrections S, T and non-oblique corrections to the Zbb coupling, the Higgs coupling precision measurements, and the current LHC direct searches.
