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 Bi24+), which we here
ascribe to Peierls-type distortions. We suggest that the Bi24+ 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.