The Weak 3D Topological Insulator Bi₁₂ Rh₃ Sn₃ I₉

Topological insulators (TIs) gained high interest due to their protected electronic surface states that allow dissipation-free electron and information transport. In consequence, TIs are recommended as materials for spintronics and quantum computing. Yet, the number of well-characterized TIs is rather limited. To contribute to this field of research, we focused on new bismuth-based subiodides and recently succeeded in synthesizing a new compound Bi₁₂ Rh₃ Sn₃ I₉ , which is structurally closely related to Bi₁₄ Rh₃ I₉ - a stable, layered material. In fact, Bi₁₄ Rh₃ I₉ is the first experimentally supported weak 3D TI. Both structures are composed of well-defined intermetallic layers of _∞ ² [(Bi₄ Rh)₃ I]²⁺ with topologically protected electronic edge-states. The fundamental difference between Bi₁₄ Rh₃ I₉ and Bi₁₂ Rh₃ Sn₃ I₉ lies in the composition and the arrangement of the anionic spacer. While the intermetallic 2D TI layers in Bi₁₄ Rh₃ I₉ are isolated by _∞ ¹ [Bi₂ I₈ ]^2- chains, the isoelectronic substitution of bismuth(III) with tin(II) leads to _∞ ² [Sn₃ I₈ ]^2- layers as anionic spacers. First transport experiments support the 2D character of this material class and revealed metallic conductivity.