Extremely low lattice thermal conductivity in light-element solid materials

Lattice thermal conductivity (<i>κ</i> _l) is of great importance in basic sciences and in energy conversion applications. However, low-<i>κ</i> _l crystalline materials have only been obtained from heavy elements, which typically exhibit poor stability and possible toxicity. Thus, low-<i>κ</i> _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 <i>κ</i> _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 <i>κ</i> _l. For example, KCu₄Se₃ exhibits an extremely low <i>κ</i> _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-<i>κ</i> _l solids comprising light elements.