Stabilizing Cubic GeSe via Metavalent Alloying for Enhanced Thermoelectric Performance

Strong covalent bonding and low crystal symmetry intrinsically constrain the thermoelectric performance of orthorhombic GeSe. Here, we introduce a synergistic approach combining metavalent alloying and vacancy engineering to stabilize a high-symmetry cubic phase and activate the latent thermoelectric potential of GeSe. Incorporation of a small amount of nonstoichiometric Bi₂Te₃ (10%) induces a structural transition from covalently bonded orthorhombic GeSe to a metavalently bonded cubic counterpart. However, the large atomic size mismatch between Bi and Ge restricts the Bi₂Te₃ solubility, causing Bi₂Se₂Te impurity precipitation and a detrimental cubic-to-hexagonal phase transition at elevated temperatures. This high-temperature structural instability is effectively suppressed by trace Cd doping, which promotes impurity redissolution and stabilizes the cubic phase across the entire temperature range. Relative to the pristine orthorhombic phase, the stabilized cubic GeSe exhibits a narrowed bandgap, sharpened band edges, enhanced band degeneracy, softened chemical bonding, increased lattice anharmonicity, and hierarchical defect structures, collectively enabling collaborative optimization of electrical and thermal transport. Consequently, (Ge_0.95Cd_0.05Se)_0.9(Bi₂Te₃)_0.1 delivers a peak zT of 0.64 at 723 K, a 13-fold enhancement over pristine GeSe. This work identifies metavalent alloying coupled with vacancy engineering as an effective and general pathway for stabilizing high-symmetry cubic phase and enhancing thermoelectric performance in low-symmetry chalcogenides.