Anti-bonding mediated record low and comparable-to-air lattice thermal conductivity of two metallic crystals

Abstract

In most pure metals and metallic systems, although the electronic thermal conductivity is believed to dominate thermal transport, the magnitude of lattice (phononic) thermal conductivity (LTC) is usually not negligibly low. We report two exceptional metallic materials, namely cubic half-Heusler-type PbAuGa and CsKNa, by solving the phonon Boltzmann transport equation (BTE) based on first-principles calculations. The two crystals possess record low LTCs of 0.064 W mK⁻¹ and 0.031 W mK⁻¹ at room temperature, respectively, among all pure metals and metallic systems we have known so far. Such LTCs, which are even comparable to that of air (about 0.025 W mK⁻¹ under ambient conditions), only contribute 0.37% and 0.29% to the overall thermal transport. By quantitatively characterizing both phonon–phonon and phonon–electron interactions, it is demonstrated that the anomalously low LTC stems from low group velocity and strong anharmonicity which can be traced back to anti-bonding nature at the electronic level. The acoustic modes of PbAuGa and CsKNa originating from Au and Cs respectively dominate the thermal transport. By examining the mean square displacement, potential energy well, and crystal orbital Hamiltonian population, we find that the magnitude of movement of loosely bonded Au and Cs atoms in PbAuGa and CsKNa respectively is appreciable, and the Au and Cs atoms act as intrinsic rattlers and thus induce strong phonon anharmonicity and ultrashort lifetime reaching the Ioffe–Regel limit. This study deepens our understanding of heat conduction in metals and metallic systems and provides a route for searching for novel materials with nearly zero lattice thermal conductivity for future emerging applications.