Crystal-liquid duality driven ultralow two-channel thermal conductivity in -MgAgSb

The desire for intrinsically low lattice thermal conductivity (kappa(L)) in thermoelectrics motivates numerous efforts on understanding the microscopic mechanisms of heat transport in solids. Here, based on theoretical calculations, we demonstrate that alpha-MgAgSb hosts low-energy localized phonon bands and avoided crossing of the rattler modes, which coincides with the inelastic neutron scattering result. Using the two-channel lattice dynamical approach, we find, besides the conventional contribution (similar to 70% at 300 K) from particlelike phonons propagating, the coherence contribution dominated by the wavelike tunneling of phonons accounts for similar to 30% of the total kappa(L) at 300 K. By considering dual contributions, our calculated room-temperature kappa(L) of 0.64 W m(-1 )K(-1) well agrees with the experimental value of 0.63 W m(-1 )K(-1). More importantly, our computations give a nonstandard kappa(L) proportional to T-0.61 dependence, perfectly explaining the abnormal temperature-trend of similar to T-0.57 in experiment for alpha-MgAgSb. By molecular dynamics simulation, we reveal that the structure simultaneously has soft crystalline sublattices with the metavalent bonding and fluctuating liquid-like sublattices with thermally induced large amplitude vibrations. These diverse forms of chemical bonding arouse mixed part-crystal part-liquid state, scatter strongly heat-carrying phonons, and finally produce extremely low kappa(L). The fundamental research from this study will accelerate the design of ultralow-kappa(L) materials for energy-conversion applications.