Chemical Bond Covalency‐Driven Cubic Phase Stabilization Boosts Thermoelectric Efficiency in Cu ₂ GeSe ₃

ABSTRACT We report a breakthrough in the thermoelectric performance of Cu 2 GeSe 3 achieved through covalent bond engineering via controlled Ge substitution for Se, which induces an orthorhombic‐to‐cubic phase transition by strengthening Cu‐Ge and Ge‐Ge covalent interactions. Comprehensive structural characterizations (XRD, HRTEM, HAADF‐STEM, XPS) combined with first‐principles calculations demonstrate that the reduced electronegativity difference and enhanced covalent bonding (57% increase in Cu─Ge bond strength, integrated COHP: 0.162 vs. 0.103) collectively suppress cation ordering and stabilize the cubic phase. The structural transformation simultaneously optimizes electronic transport through valence band convergence at Γ and X points and bandgap reduction, leading to improved Seebeck coefficient and carrier mobility. Concurrently, spontaneous Ge vacancies and cation disorder synergistically suppress lattice thermal conductivity to 0.48 W m −1 K −1 at 773 K. The optimized cubic‐phase Cu 2 GeSe 2.7 Ge 0.3 achieves a peak ZT of 0.65 at 773 K, representing a substantial improvement over pristine orthorhombic samples and surpassing all reported singly‐doped Cu 2 GeSe 3 systems. This work establishes covalent bond engineering as a universal strategy for diamond‐like chalcogenides (e.g., Cu 2 SnSe 3 , CuInSe 2 ), enabling decoupled optimization of electronic and thermal transport through targeted orbital hybridization.