Modulating phonon dynamics: tailoring lattice vibrations to enhance thermoelectric efficiency in Mg₃(Sb, Bi)₂ alloy

Heat in crystalline materials is transported by phonons from lattice vibrations, and lattice thermal conductivity critically determines thermoelectric performance. Different from conventional approach that reduce thermal conductivity via extrinsic additives sacrificing electrical transport, here, we demonstrate a notable advancement in the n-type Mg₃Sb_1.5Bi_0.5 by modulating phonon dynamics through lattice softening and simultaneously suppressing the phonon mean free path in a more localized manner while remaining compositionally invariant. Originating from Mg vacancies and derivative defects, elevated internal strain degrades bonding rigidity and localize phonons at the lattice-constant level, yielding an ultra-low thermal conductivity of 0.3 W m⁻¹ K⁻¹, close to the theoretical minimum. This intrinsic strategy, combined with electron concentration optimization, yields a ZT_max of 2.06 and an extraordinary ZT_ave of 1.58, exceeding state-of-the-art n-type materials. Furthermore, a single-leg generator and two-pair module deliver conversion efficiencies of 12.5% (ΔT = 440 K) and 7.4% (ΔT = 300 K), respectively, highlighting exceptional potential for waste heat recovery.