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 the
Bi13S18X2 (in which X = Br and I) structure. Such a model structure displays columns of Bi atoms that form dimers of subvalent Bi (formally
Bi2 4+) which we here ascribe to Peierls-type distortions. We suggest that the Bi2 4+ dimers are at the origin of the anomalously low band gap of this material, with the lowest energy electronic transition showing analogies with an intervalence charge transfer. Our synthetic approach and insights into the structural and electronic features of the hexagonal bismuth chalcohalides are fundamental to sustain the
rapidly increasing use of this class of (nano)materials for diverse applications, such as photocatalysis and thermoelectrics.