Role of Covalent Cages and Rattler Atoms in Lowering the Thermal Conductivity in Zintl Metal Chalcogenides

Zintl phases represent a class of compounds, mainly intermetallics, which are characterized by ionic and covalent bonds in the same crystal. Since its discovery in the late 1800s, Zintl phases have found their importance as an academic interest due to their fascinating structure as well as in industry due to their vast applicability. In recent years, the Zintl phase of metal chalcogenides has further demonstrated its ability as a promising thermoelectric material, primarily due to its intrinsically ultralow lattice thermal conductivity (κlat). In this viewpoint, we discuss the origin of ultralow κlat in Zintl metal chalcogenides. Our viewpoint illustrates how the characteristic structural features and chemical bonding hierarchy in Zintl phases play a pivotal role in suppressing heat transport and why Zintl metal chalcogenides are one of the promising candidates for future high-performance thermoelectrics.