**1.4 Cs2SnI6-typed perovskite**

A new class of perovskite variants *A2BX6* would be attractive candidates because *B* in *A2BX6* is expected to be the +4 oxidation state upon assumption of the A+ and X ion states, which would lead to a stable structure in air and moisture [8]. It can be inferred that the electronegativity and ionic radii play an important role to be the stable structure, the lattice parameters of which are determined by the competition between the ionic Coulomb and exchange correlation contributions [17]. This typed crystals are *Fm-3 m* space group with four formula units in one unit cell, and it is chemically bonded from the Coulomb interactions between the particular ions [17]. In this crystalline structure, 12-fold of each monovalent cation A coordinates (in the unit of the lattice constant) are (0.25, 0.25, 0.25), each tetravalent cation B is sixfold coordinated by the halogen ions (*x*, 0, 0), where *x* is somewhat different (varying around 0.2) for different structures. From optical band structure, halogen ions are located in the upper valence band and the bottom of conduction bands is formed dominantly by the cationic *s*-states. Usually incorporation of the d-transition metal ions (in the case of

doped crystals) gives additional localized *d*-states, which form the bottom of the conduction bands.

### **1.5 Characteristics of Cs2SnI6 perovskite**

#### *1.5.1 Structural properties*

The α-phase of Cs2SnI6 crystalizes into the face-centered-cubic (fcc) K2PtCl6 type with the space group *Fm*3̅*m* (the antifluorite structure) and the lattice parameter a of 11.6276 (9) [16]. The unit cell is configured of four {SnI6} <sup>2</sup>�octahedra at the corners and the face centers and eight Cs2+ cations at the tetragonal interstitials. (see **Figure 1(b)**) Alternatively, Cs2SnI6 can be regarded as a defective variant of the AMX3 structure type similar to those of CsSnI3, CH3NH3SnI3 and CH3NH3PbI3 compounds, in which the {SnI6} octahedra connect to each other by sharing their corners. In Cs2SnI6, half of the octahedral *Sn* atoms are missing creating discrete {SnI6} 2� octahedra. The compound is therefore a molecular salt and contains Sn4+ rather than Sn2+ in CsSnI3. This accounts of the stability of the material. After the half of the Sn atoms are removed, the {SnI6} <sup>2</sup>� octahedra shrink slightly, leading to the smaller Sn � I bond length (2.85 Å) [17] in Cs2SnI6 than that in CsSnI3 (3.11 Å) [9] as well as the smaller intraoctahedral I � I bond length (4.04 Å) than that of interoctahedral I � I <sup>0</sup> bond lengths (4.20 Å).
