Layered Ordered Multianion Semiconductor Bi₁₈O_21.6Se_1.8Cl_7.2 with Low <i>c</i>-Axial Thermal Conductivity

Layered ordered multianion materials exhibit remarkable structural flexibility and unique chemical and physical properties, which arise from the interplay of intralayer and interlayer interactions, as well as distinct local chemical environments originating from different anionic sublattices. Here, we report a novel layered quaternary compound, Bi₁₈O_21.6Se_1.8Cl_7.2, which features three different anionic sublattices. Bi₁₈O_21.6Se_1.8Cl_7.2 consists of [Bi₆O_9.6Cl₆] and [Bi₁₂O₁₂Se_1.8Cl_1.2] slabs that stack along the <i>c</i> axis alternatively. Comprehensive physical properties and electronic properties of Bi₁₈O_21.6Se_1.8Cl_7.2 single crystals reveal semiconducting behavior with an indirect band gap of ∼1.51 eV and the dominant electron-type carriers. Notably, Bi₁₈O_21.6Se_1.8Cl_7.2 exhibits exceptionally low <i>c</i>-axial thermal conductivity κ_c (∼0.261-0.307 W m^-1 K^-1) at room temperature, significantly expanding the phase space of the ultralow-thermal-conductivity Bi-O-Se-Cl system. Our findings highlight that the strategic combination of distinct two-dimensional building blocks with varied structural and anionic coordination environments offers an effective approach for designing new layered materials with tunable physical properties.