Due to the growing demand for sustainable energy, thermoelectric (TE) technology that directly converts unserviceable waste heat into available electricity is gaining traction. [1][2][3][4] However, the widespread use of TE technology is severely hampered by its relatively low energy conversion efficiency, [5] which depends on the composed material's figure of merit zT ¼ κα 2 T/κ, where α, σ, κ, and T are the Seebeck coefficient, electrical conductivity, total thermal conductivity (including the lattice contribution κ L and the carrier contribution κ e ), and absolute temperature, respectively. [2,3,6] However, in view of the sophisticated competitive relationship between α, σ, and κ, it is currently an overwhelming challenge to achieve considerable progress in zT.Recently, the orthorhombic SnSe has received extensive attention due to its ultrahigh zT % 2.8. [7] The remarkable lattice anharmonicity evoked by the interestingly layered structure of SnSe is the main reason for its ultralow κ L and distinguished zT. [8] In consideration of the similar crystal

Composite engineering favors high thermoelectric performance by tuning the carrier and phonon transport. Herein, orthorhombic and rhombohedral dualphase GeSe are designed in situ by tailoring chemical bonds. Atom probe tomography verifies the coexistence of a covalently bonded orthorhombic phase and a metavalently bonded rhombohedral phase in GeSe-InTe alloys. The production of the rhombohedral phase simultaneously increases the carrier concentration, the carrier mobility, the band degeneracy, and the density-of-states effective mass due to the reduced formation energy of cation vacancies and the improved crystal symmetry. These attributes are beneficial to a high-power factor. In addition, the thermal conductivity can be significantly reduced due to the intrinsically strong lattice anharmonicity of the metavalently bonded phase, the interfacial acoustic phonon mismatch across different bonding mechanisms, and the phonon scattering at vacancy-solute clusters. Moreover, the metavalently bonded phase embraces higher solubility of dopants that enables the further optimization of properties by Cd-Ag doping, resulting in a zT of 0.95 at 773 K as well as enhanced strength and ductility in dual-phase Ge 0.94 Cd 0.03 Ag 0.03 Se(InTe) 0.15 . This work indicates that in situ design of dual-phase composites by tailoring chemical bonds is an effective method for enhancing the thermoelectric and mechanical properties of GeSe and other p-bonded chalcogenides.