Abstract
Lattice thermal conductivity (κ (l)) is of great importance in basic sciences and in energy conversion applications. However, low-κ (l) crystalline materials have only been obtained from heavy elements, which typically exhibit poor stability and possible toxicity. Thus, low-κ (l) materials composed of light elements should be explored. Herein, light elements with hierarchical structures in a simple square-net lattice as well as a small discrepancy in atomic mass and radius exhibit low κ (l). The hierarchical structure exhibits various chemical bonds and asymmetric geometry of building units, resulting in flat phonon branches and strong phonon-phonon interactions similar to those observed in heavy-element materials. These phenomena generate a large phonon anharmonicity, which is the prerequisite for achieving extremely low κ (l). For example, KCu(4)Se(3) exhibits an extremely low κ (l) of 0.12 W/(m·K) at 573 K, which is lower than that of most heavy-element materials. These findings can reshape our fundamental understanding of thermal transport properties of materials and advance the design of low-κ (l) solids comprising light elements.