Prospective Chalcohalide Perovskites: Pursuing (and Failing) the Synthesis of CsBiSCl₂ Nanocrystals

Heavy pnictogen chalcohalides are often termed lead-free, perovskite-inspired materials. Despite theoretical predictions, incontrovertible experimental demonstrations of heavy pnictogen chalcohalides adopting a perovskite structure are lacking. Here we report our attempts to prepare CsBiSCl₂ adopting a perovskite structure as colloidal nanocrystals. Synthesis of nanoscale materials can indeed rely on fast, nonequilibrium reactions and on large, eventually thermodynamically favorable surface energies, leading to the possibility of stabilizing kinetically trapped or metastable phases. However, we obtained no CsBiSCl₂, but a mixture of nanocrystals of secondary phases, namely Cs₃BiCl₆ submicrometric polyhedra, Bi₂S₃ nanoscopic rods, and Cs₃Bi₂Cl₉ nanoscopic dots, whose low polydispersity enabled an effective separation via size/shape selective precipitation. This work confirms that heavy pnictogen chalcohalides are hardly prone to adopting a perovskite structure. Nevertheless, chemistry at the nanoscale offers multiple possibilities for overcoming phase segregation and pursuing the synthesis of prospective mixed anion compound semiconductors.