Amorphous-like Thermal Conductivity in Crystalline Solids

Thermoelectric materials (TEs) could play an important role in future energy management through environmentally sound cooling and power generation, e.g., converting waste heat into electricity. Efficient TEs inhibit the propagation of heat (low thermal conductivity, κ) but conduct electricity well (high power factor). Although κ in a given material can be reduced via alloying and nanostructuring, identifying materials with intrinsically low κ is still needed. Previously, it has been shown that soft phonon modes due to weakly bonded atoms and s² lone-pair electrons are common to materials with low-κ. Here, we predict a series of new materials which are weakly bonded systems with same constituent elements but different stoichiometry either with s² lone-pair electrons or high mass density. Due to resulting giant phonon anharmonicity and low phonon group velocities, they offer extremely low κ (0.3-0.6 W/mK) at 300K approaching those found in the amorphous/disordered regime. In addition to low-κ, high Seebeck coefficients and high electrical conductivities in these materials may provide a new opportunities for high-efficiency thermoelectrics at room temperature.