Band Gaps and Stability of CsSiX3 Halides

There is a great current interest in lead‐free halide perovskites. While Sn based and Ge based compounds of this type have already been demonstrated experimentally, Si based ones have not. In this paper, the authors consider the possibility of CsSiX 3 halide perovskites. While previous work for cubic CsSiI 3 found the band gap to close or in fact become inverted and to lead to a topological insulator, the authors show here that in the rhombohedrally distorted structure that occurs because of Si off‐centering, the band gap becomes larger than 1.5 eV and possibly of interest for photovoltaics. On the other hand, even in the cubic structure at high temperature, fluctuations of the Si position, will lead to a sizable gap. The total energy calculations, show that the materials are unstable toward 3CsSiI 3 → Si 2 I 6 + 3CsI + Si and 2CsSiI 3 → SiI 4 + 2CsI + Si and are thus above the convex hull. Similar results are presented for halogens: I, Br, Cl.