Mixed Valence of Bismuth in Hexagonal Chalcohalide Nanocrystals

We report the colloidal synthesis of bismuth chalcohalide nanocrystals, which adopt a hexagonal phase that we describe with Bi13S18X2 (in X = Br and I) structure. Such model structure displays columns Bi atoms form dimers subvalent (formally Bi24+), here ascribe to Peierls-type distortions. suggest Bi24+ are at origin anomalously low band gap this material, lowest energy electronic transition showing analogies an intervalence charge transfer. Our synthetic approach insights into structural features chalcohalides fundamental sustain rapidly increasing use class (nano)materials for diverse applications, such as photocatalysis thermoelectrics.