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 Cu8S10S4/2 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.