Low-Alloying Entropy Engineering Stabilizes Cubic GeSe for Enhanced Thermoelectric Performance

The thermoelectric potential of orthorhombic GeSe is markedly constrained by its pronounced intrinsic covalent bonding alongside low structural symmetry. Here, we report a high-entropy alloying strategy using AgSnSbTe3 to activate the latent thermoelectric potential of GeSe by stabilizing a high-symmetry cubic phase. Incorporating a minimal fraction of AgSnSbTe3 triggers a lattice phase transformation from orthorhombic GeSe, which features covalent bonding, to a cubic structure governed by metavalent bonding. Compared with conventional I–V–VI2 alloying approaches that typically require high dopant concentrations, the high-entropy AgSnSbTe3 enables efficient phase engineering at significantly lower composition levels. The elevated configurational entropy markedly expands the solid solubility of constituent elements, inhibits phase segregation, and promotes the persistence of a uniform, single-phase cubic solid solution across a broad temperature range. The stabilized cubic GeSe exhibits significantly weakened chemical bonding, reduced cation vacancy formation energy, enhanced band degeneracy, increased lattice anharmonicity, and multiscale defect structures, which together allow electrical and thermal transport properties to be optimized synergistically. As a result, (GeSe)0.75(AgSnSbTe3)0.25 achieves a peak zT of 0.92 at 723 K, representing a substantial improvement over pristine GeSe. This work demonstrates that entropy-driven alloying with AgSnSbTe3 provides an effective and general strategy to stabilize high-symmetry cubic phases and boost thermoelectric performance in low-symmetry chalcogenide systems.