Concerted Rattling in CsAg5Te3 Leading to Ultralow Thermal Conductivity and High Thermoelectric Performance

Thermoelectric (TE) materials convert heat energy directly into electricity, and introducing new materials with high conversion efficiency is a great challenge because of the rare combination of interdependent electrical and thermal transport properties required to be present in a single material. The TE efficiency is defined by the figure of merit ZT=(S2σ) T/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the total thermal conductivity, and T is the absolute temperature. A new p‐type thermoelectric material, CsAg5Te3, is presented that exhibits ultralow lattice thermal conductivity (ca. 0.18 Wm−1 K−1) and a high figure of merit of about 1.5 at 727 K. The lattice thermal conductivity is the lowest among state‐of‐the‐art thermoelectrics; it is attributed to a previously unrecognized phonon scattering mechanism that involves the concerted rattling of a group of Ag ions that strongly raises the Grüneisen parameters of the material.