Layered Ordered Multianion Semiconductor Bi18O21.6Se1.8Cl7.2 with Low c-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, Bi18O21.6Se1.8Cl7.2, which features three different anionic sublattices. Bi18O21.6Se1.8Cl7.2 consists of [Bi6O9.6Cl6] and [Bi12O12Se1.8Cl1.2] slabs that stack along the c axis alternatively. Comprehensive physical properties and electronic properties of Bi18O21.6Se1.8Cl7.2 single crystals reveal semiconducting behavior with an indirect band gap of ∼1.51 eV and the dominant electron-type carriers. Notably, Bi18O21.6Se1.8Cl7.2 exhibits exceptionally low c-axial thermal conductivity κc (∼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.