Lone-Pair Electron-Driven Thermoelectrics at Room Temperature

Identifying materials with good electron transport and poor thermal transport properties for thermoelectric applications has been challenging. Here we report a series of new materials including Tl₃TaSe₄ and Tl₃VS₄ with promising thermoelectric properties giving thermoelectric figure of merit, zT ≈ 0.8 at room temperature using first-principles calculations. This high zT stems from the high electrical conductivity and ultralow thermal conductivity (κ). We calculate κ ≈ 0.1-0.2 W/m·K from a phonon Boltzmann's transport equation and κ ≈ 0.3-0.4 W/m·K from the two-channel model. Low phonon group velocities due to weakly bonded Tl atoms and strong anharmonicity associated with s² lone electrons pair give rise to such a low κ in these systems.