Rapid Solidification Brings Ultralow Thermal Conductivity and Superior Thermoelectric Performance for Intrinsic GeTe Alloy

Intrinsic GeTe suffers from excessively high carrier concentrations caused by Ge vacancies and limitations in phase regulation, which constrain further enhancement of its thermoelectric performance. In this work, rapid solidification with glass fluxing techniques is employed to achieve a large undercooling of Δ T = 112–122 K in the GeTe alloy for the first time. By Ge enrichment, the carrier concentration is tuned to an optimal range of 5.7 × 10 20 cm −3 and the carrier mobility is enhanced to 75.3 cm 2 V −1 s −1 . The stoichiometric GeTe alloy retains a portion of the high‐temperature cubic phase at room temperature, forming a rhombohedral‐cubic (α+β) uniform dual‐phase structure with a specific crystallographic orientation. DFT calculations of the band structure, density of states, and phonon spectrum reveal the influence of the α+β structure on electrical transport and thermal diffusion. This unique microstructure combined with multiscale defect engineering enables a high figure of merit ( zT ) of 1.32 at 700 K for the Ge 1.03 Te alloy, with a total processing time of less than 2 h, significantly improving the preparation efficiency. This work demonstrates that liquid undercooling technology is an efficient preparation method for effectively regulating the phase composition and microstructure of thermoelectric alloys.