Loose bonding induced ultralow lattice thermal conductivity of a metallic crystal KNaRb.

• This study presents, for the first time, the identification of an exceptional metallic material, KNaRb, characterized by an ultralow lattice thermal conductivity of 0.114 W/mK at room temperature, a value comparable to the lowest thermal conductivities reported for metals and metallic materials. • The contribution of phononic thermal conductivity to the overall thermal transport in KNaRb amounts to merely 0.66 %, markedly lower than the typical range observed in other metals and metallic systems. • The ultralow phononic thermal conductivity observed in KNaRb can be attributed to the loose bonding characteristics of Rb. In the domain of metallic systems, electronic thermal conductivity typically governs heat transfer, while lattice (phononic) thermal conductivity (LTC) remains non-negligible magnitude. This study introduces the exceptional metallic material, cubic half-Heusler-type KNaRb, via phonon Boltzmann transport equation (BTE) resolution and first-principles calculations. KNaRb exhibits an ultralow LTC of 0.114 W/mK at room temperature, comparable to the lowest reported for metallic materials, contributing merely 0.66 % to overall thermal transport. Analysis attributes KNaRb's low LTC to low group velocity and strong anharmonicity, originating from its loosely bonded electronic structure. Acoustic modes, primarily from Rb atoms, dominate thermal transport. Examination of mean square displacement and potential energy profiles reveals significant movement of loosely bonded Rb atoms within KNaRb, acting as intrinsic "rattlers," inducing pronounced phonon anharmonicity and ultrashort lifetimes. This study advances understanding of heat conduction in metals, offering insights into materials with extremely low lattice thermal conductivity for potential future applications. [ABSTRACT FROM AUTHOR]