Precipitate–vacancy coupling in Bi-doped Ge-deficient GeTe enables high thermoelectric performance

GeTe is a promising lead-free thermoelectric material, yet its optimization remains constrained by intrinsic Ge-vacancy disorder and the phase instability associated with vacancy-rich, non-stoichiometric chemistry. In Bi-doped Ge-deficient GeTe, this instability is often accompanied by the expulsion of Ge from the matrix, leading to phase separation that is usually regarded as detrimental to transport performance. Here, we show that ultra-rapid solidification can convert this instability into a beneficial microstructural feature. In Bi_0.07Ge_0.9Te, ultra-rapid solidification stabilizes a distinctive metastable microstructure consisting of dense planar vacancy sheets and nanoscale-confined Ge precipitates within the GeTe matrix. Structural and microscopic analyses reveal that rapid solidification suppresses Ge coarsening while preserving a vacancy-rich matrix state, thereby establishing precipitate–vacancy coupling in a far-from-equilibrium microstructure. This coupled architecture introduces additional multiscale phonon-scattering channels while maintaining a reasonable power factor, leading to markedly reduced lattice thermal conductivity and enhanced thermoelectric performance. As a result, the optimized Bi_0.07Ge_0.9Te sample achieves a power factor of 3.45 mW m⁻¹ K⁻² at 623 K and a peak zT of 1.91 at 673 K. These results demonstrate that precipitate–vacancy coupling offers an effective route for microstructure engineering in GeTe-based thermoelectrics.