Lone-pair Driven Thermoelectrics at Room Temperature

Identifying materials which offer crystal-like charge transport and glass-like phonon transport for improved efficiency (ZT) of thermoelectric materials has been a challenging task. The complex interrelationship between these competing materials properties makes such materials rare. Here we report a series of new materials with promising thermoelectric properties; ZT ~ 0.8, at room temperature. This superior ZT stems from the high electrical conductivity and ultralow thermal conductivity; k ~ 0.3 W/m-K, as obtained from the two-channel model (phonons + oscillators). The calculated k is mainly dominated by the Einstein oscillators, which is similar to the cases with amorphous materials where mean free paths become comparable with the characteristics atomic bond lengths. Very low-group velocities due to weakly bonded atoms and strong anharmonicity associated with S² lone-pair electrons explain such low-k in these systems.