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.
Beta-BaZn2As2 is known to be a p-type semiconductor with the layered crystal structure similar to that of LaZnAsO, leading to the expectation that beta-BaZn2As2 and LaZnAsO have similar bandgaps; however, the bandgap of beta-BaZn2As2 (previously-repo
rted value ~0.2 eV) is one order of magnitude smaller than that of LaZnAsO (1.5 eV). In this paper, the reliable bandgap value of beta-BaZn2As2 is determined to be 0.23 eV from the intrinsic region of the tem-perature dependence of electrical conductivity. The origins of this narrow bandgap are discussed based on the chemi-cal bonding nature probed by 6 keV hard X-ray photoemission spectroscopy, hybrid density functional calculations, and the ligand theory. One origin is the direct As-As hybridization between adjacent [ZnAs] layers, which leads to a secondary splitting of As 4p levels and raises the valence band maximum. The other is that the non-bonding Ba 5dx2-y2 orbitals form unexpectedly deep conduction band minimum (CBM) in beta-BaZn2As2 although the CBM of LaZnAsO is formed mainly of Zn 4s. These two origins provide a quantitative explanation for the bandgap difference between beta-BaZn2As2 and LaZnAsO.
