Modulation of transport properties via S/Br substitution: solvothermal synthesis, crystal structure, and transport properties of Bi 13 S 17 Br 3 .

The solvothermal synthetic exploration of the Bi-S-halogen phase space resulted in the synthesis of two bismuth sulfohalides with common structural motifs. Bi 13 S 18 I 2 was confirmed to have the previously reported composition and crystal structure. In contrast, the bromide analogue was shown to have a formula of neither Bi 19 S 27 Br 3 nor Bi 13 S 18 Br 2 , in contrast to the previous reports. The composition, refined from single crystal X-ray diffraction and confirmed by elemental analysis, high-resolution powder X-ray diffraction, and total scattering, is close to Bi 13 S 17 Br 3 due to the partial S/Br substitution in the framework. Bi 13 S 18 I 2 and Bi 13 S 17 Br 3 are n -type semiconductors with similar optical bandgaps of ∼0.9 eV but different charge and heat transport properties. Due to the framework S/Br disorder, Bi 13 S 17 Br 3 exhibits lower thermal and electrical conductivities than the iodine-containing analogue. The high Seebeck coefficients and ultralow thermal conductivities indicate that the reported bismuth sulfohalides are promising platforms to develop novel thermoelectric materials.