Extremely low lattice thermal conductivity in light-element solid materials.

Lattice thermal conductivity ( ) is of great importance in basic sciences and in energy conversion applications. However, low- crystalline materials have only been obtained from heavy elements, which typically exhibit poor stability and possible toxicity. Thus, low- 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 . 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 . For example, KCuSe exhibits an extremely low 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- solids comprising light elements.