Additive-assisted synthesis and optoelectronic properties of (CH₃NH₃)₄Bi₆I₂₂

Hybrid organic-inorganic halides containing Bi and Sb generally exhibit higher stability and lower toxicity compared to Pb analogues. In this work, the synthesis, crystal and electronic structures and optical properties of a brand-new methylammonium bismuth iodide, (MA)₄Bi₆I₂₂ (MA⁺ = CH₃NH₃⁺), are reported. Interestingly, we find that the presence of the HgI₂ is necessary for the targeted preparation of (MA)₄Bi₆I₂₂. (MA)₄Bi₆I₂₂ contains isolated [Bi₆I₂₂]^4- clusters made of six edge-sharing octahedral BiI₆ units, which are separated by MA⁺ cations in its 0D crystal structure. A relatively low optical band gap of 1.9 eV was estimated for (MA)₄Bi₆I₂₂ based on diffuse reflectance measurements. An intense photoluminescence peak emerges at 636 nm at low temperatures that supports the assigned band gap value. Electronic structure calculations show the presence of flat bands in the valence and conduction bands, consistent with the low-dimensional structure of (MA)₄Bi₆I₂₂, and slightly indirect nature of the bandgap. Our findings suggest that the use of facilitator moieties such as HgI₂ may provide a pathway to obtaining alternative methylammonium bismuth iodides to (MA)₃Bi₂I₉.