Charge Transfer Driven Symmetry Stabilization Enables Scalable High Efficiency GeTe Thermoelectrics

The practical deployment of thermoelectric materials is often limited by the trade‐off between high performance and structural stability, as lattice instabilities that suppress thermal transport frequently induce phase transitions and mechanical degradation. Here, we demonstrate a charge‐transfer‐driven strategy to stabilize crystal symmetry in GeTe‐based TAGS‐85 thermoelectrics. Through bonding‐guided multi‐element alloying, enhanced charge transfer suppresses the Peierls distortion and completely eliminates the rhombohedral‐to‐cubic phase transition, locking the cubic phase from room temperature to 800 K. Simultaneously, increased lattice complexity promotes acoustic‐optical phonon hybridization, leading to strong phonon scattering and ultralow lattice thermal conductivity. As a result, the optimized (Ge 0.83 Pb 0.12 Bi 0.01 Sn 0.04 Te) 0.85 (AgSbTe 2 ) 0.15 alloy achieves a peak zT of 2.64 and an average zT of 1.83 over 300–800 K, together with significantly enhanced mechanical robustness. The stabilized cubic structure further enables the scalable fabrication of crack‐free bulk ingots up to 40 mm in diameter and 200 g in mass. Thermoelectric devices based on this material deliver a conversion efficiency of 14% and a maximum cooling temperature difference of 58 K. These results establish charge‐transfer‐driven symmetry stabilization as an effective route toward high‐performance, mechanically robust, and manufacturable thermoelectric materials.