Bi₁₃S₁₈I₂: (Re)discovery of a Subvalent Bismuth Compound Featuring [Bi₂]⁴⁺ Dimers Grown in Sulfur/Iodine Flux Mixtures

Reactions of bismuth in sulfur/iodine flux mixtures were explored at various temperatures and iodine concentrations. The observed products include Bi2S3, BiSI, and Bi13S18I2. The latter compound, formerly reported as “Bi19I3S27” grows as well formed needles from the flux. This enabled extensive crystallographic studies by single crystal XRD and powder synchrotron XRD. These data allow a more accurate assignment of the disordered bismuth sites in the structure, indicating formation of subvalent Bi24+ dimers; this results in a stoichiometry of Bi13S18I2. Thermal decomposition studies and Raman spectroscopy support this structural model, and electronic structure calculations and optical reflectance studies indicate this compound is an indirect band gap semiconductor with band gap of 0.3 eV.