Expanding multiple anion superlattice chemistry: Synthesis, structure and properties of Bi4O4SeBr2 and Bi6O6Se2Cl2

The synthesis, structure, and properties of the three-anion superlattice materials Bi4O4SeBr2 and Bi6O6Se2Cl2 are reported. These materials crystallise in structures that form part of a homologous series of compounds comprised of stackings of BiOCl- and Bi2O2Se-like units. Bi4O4SeBr2 is analogous to Bi4O4Se2Cl2, whereas Bi6O6Se2Cl2 contains an additional Bi2O2Se layer that produces off-centred anions. The band gaps of both materials are indirect, with Eg ​= ​1.15(5) eV, and the materials behave as doped semiconductors with very low thermal conductivities. These materials expand the synthetic scope of multiple anion superlattice materials and, with optimisation, may also be platforms for future thermoelectric materials. Two new materials have been synthesized and characterised, expanding on the multiple anion superlattice chemistry of Bi4O4SeCl2. A second Bi2O2Se unit can be added, resulting in Bi6O6Se2Cl2, or the Cl can be replaced by Br, resulting in Bi4O4SeBr2. The change in stacking in Bi6O6Se2Cl2 results in off-centred anions, as shown by the black arrows. These materials are both n-type semiconductors with very low thermal conductivities. [Display omitted] •The layered materials with compositions Bi4O4SeBr2 and Bi6O6Se2Cl2 are synthesized and characterised.•Bi4O4SeBr2 and Bi6O6Se2Cl2 belong to a three anion homologous series.•Bi4O4SeBr2 and Bi6O6Se2Cl2 are semiconductors with band gaps in the visible light spectrum.•Bi4O4SeBr2 and Bi6O6Se2Cl2 exhibit very low thermal conductivities.•Three anion materials Bi4O4SeBr2 and Bi6O6Se2Cl2 demonstrate structural diversity through symmetry and layering sequences.