Ba₅Cu₈In₂S₁₂: a quaternary semiconductor with a unique 3D copper-rich framework and ultralow thermal conductivity

A novel quaternary sulfide, Ba₅Cu₈In₂S₁₂ (1), has been successfully synthesized via a high-temperature solid-state reaction. It contains Cu₈S₁₀S_4/2 clusters as basic building blocks, which are connected to one another by discrete In³⁺ ions to generate a 3D copper-rich framework, where the Ba²⁺ cations reside. Interestingly, such large clusters that are fused by five crystallographically independent Cu sites with three different chemical environments result in the increase of phonon scattering, which is the crucial factor to the exceptionally low lattice thermal conductivity (ca. 0.28 W m^-1 K^-1 at 773 K) in 1.