Enhanced Thermoelectric Performance and Low Thermal Conductivity in Cu₂GeTe₃ with Identified Localized Symmetry Breakdown

Highly efficient and eco-friendly thermoelectric generators rely on low-cost and nontoxic semiconductors with high symmetry and ultralow lattice thermal conductivity κ_L. We report the rational synthesis of the novel cubic (Ag, Se)-doped Cu₂GeTe₃ semiconductors. A localized symmetry breakdown (LSB) was found in the composition of Cu_1.9Ag_0.1GeTe_1.5Se_1.5 (i.e., CAGTS15) with an ultralow κ_L of 0.37 W/mK at 723 K, the lowest value outperforming all Cu₂GeCh₃ (Ch = S, Se, and Te). A joint investigation of synchrotron X-ray techniques identifies the LSB embedded into the cubic CAGTS15 host matrix. This LSB is an Ångström-scale orthorhombic symmetry unit, characteristic of multiple bond lengths, large anisotropic atomic displacements, and distinct local chemical coordination of anions. Computational results highlight that such an unusual orthorhombic symmetry demonstrates low-frequency phonon modes, which become softer and more predominant with increasing temperatures. This unconventional LSB promotes bond complexity and phonon scattering, highly beneficial for extraordinarily low lattice thermal conductivity.