Glassy Thermal Transport Triggers Ultra‐High Thermoelectric Performance in GeTe

Abstract The consequences of broken long‐range atomic arrangement in glasses or amorphous solids are reflected in the temperature dependence of lattice thermal conductivity (κ_lat). However, the appearance of glassy ultralow κ_lat in a crystalline solid with high electrical transport like metal is unusual but can have a remarkable impact on the thermoelectric performance of a material. Here, an ultra‐high thermoelectric performance is demonstrated with a maximum figure of merit, zT ≈ 2.7 (≈2.92 with Dulong–Petit heat capacity) via achieving glassy thermal transport along with significant electrical conductivity in ball milled BiSe, Pb co‐doped polycrystalline Ge_1.03Te followed by spark plasma sintering. The glassy thermal transport results from the inhomogeneous ferroelectric instability developed due to local polar distortions near the dopant sites, which interacts with soft polar optical modes via strain fluctuations. Resulting structural degeneracy and associated soft vibrations sink heat effectively from acoustic phonons, which along with various nanoscale defects, confine the phonon mean free path (MFP) close to the interatomic distance, rendering the thermal transport glassy. However, the material still maintains a high electrical conductivity at ambient condition due to much longer MFP of the charge carriers. A promising output power density of ≈0.8 W cm⁻² for ΔT ≈441 K in double‐leg thermoelectric device demonstrate the potential of this material for mid‐temperature thermoelectric applications.