Intrinsic low thermal conductivity in weakly ionic rocksalt structures

A fundamental challenge in thermoelectric (TE) material research is meeting the simultaneous requirements of high carrier mobility and low thermal conductivity. Simple crystal structures are ideal for maintaining high carrier mobility, but they usually have high thermal conductivity. Here we show by first-principles lattice dynamics and Boltzmann transport calculations that weakly ionic rocksalt structures exhibit strong lattice anharmonicity and low acoustic-phonon group velocity, which combine to produce intrinsic low thermal conductivity. We unveil microscopic mechanisms that explain experimental observations and provide insights for TE material design and discovery.