Crystal Structure Induced Ultralow Lattice Thermal Conductivity in Thermoelectric Ag_9AlSe_6

摘要

Recent discoveries of novel thermoelectric materials largely rely on an intrinsic low lattice thermal conductivity. This results from various mechanisms including low sound velocity, complex crystal structure, liquid-like ions, and lattice anharmonicity. Here semiconducting Ag9AlSe6 with many weakly bonded and highly disordered cations is shown to be a promising novel thermoelectric material, due to its ultralow lattice thermal conductivity (kappa(L)) of approximate to 0.3 W m(-1) K-1 in the entire temperature range. Such a low kappa(L) is believed to be a result of its (1) complex crystal structure for a small population of acoustic phonons, (2) soft bonding for an overall low sound velocity (1300 m s(-1)), and (3) massive disordering of Ag ions. Its electronic transport properties can be well understood by a single parabolic band model with acoustic scattering. The achieved thermoelectric figure of merit (zT) can be as high as unity, which is unlike conventional thermoelectric materials, which rely heavily on a high power factor. This work not only demonstrates Ag9AlSe6 as a promising thermoelectric material, but also paves the way for the exploration of novel thermoelectrics with a complex crystal structure with weakly bonded and highly disordered constituent elements in the structure.