Origins of ultralow thermal conductivity in mixed-ion-electron conductor KAg₂SbS₄

Promising thermoelectric materials exhibit high electrical conductivity and low thermal conductivity. Recent reports of ternary and quaternary diamond-like semiconductors (DLSs) that exhibit low thermal conductivity while maintaining their high electrical conductivity have sparked interest in DSLs for potential thermoelectric candidates. Here we present the experimental properties of another promising DSL compound, KAg₂SbS₄, which exhibits exceptionally low lattice thermal conductivity (0.55 W/mK). In the present work we employ experimental methods (namely XRD, resonance ultrasound spectroscopy, and impedance spectroscopy) and density functional theory (DFT) theory to understand the structural and ionic origins of the ultra-low thermal conductivity in KAg₂SbS₄. Phonon calculations suggest that the ultralow thermal conductivity in this material arises from the severely distorted tetrahedral coordination environment of the K atoms, as well as soft, low velocity modes associated with Ag displacement. Additionally, we demonstrate that KAg₂SbS₄ exhibits high ionic conductivity, which may also be responsible for additional phonon-scattering in this material.