Lattice Overdamping Induced Anisotropy Decoupling of Phonon and Carrier Transports in Quasi‐1D KCu 7 S 4 Textured Materials

Abstract A diversity of inorganic semiconductors with quasi‐low‐dimensional structures are promising thermoelectrics due to their intrinsically low lattice thermal conductivity. However, electrical and thermal conductions for such materials are commonly facilitated by the same preferential microstructural orientation, hindering the improvement of thermoelectric performance. Herein, higher electrical conductivity yet lower lattice thermal conductivity (e.g., 0.48 W m −1 K −1 at 721 K) is demonstrated in polycrystalline, quasi‐1D KCu 7 S 4 along the direction perpendicular to pressing (possessing texturing along the 1D chains). Theoretical calculations based on the unified phonon transport theory reveal that the wave‐like coherences play a dominant role in the overdamped phonon transport and in turn alter the conventional anisotropy of lattice thermal conductivity, which originates from the strong rattling anharmonicity of loose, tilted Cu–S triangular coordination, narrow phonon inter‐band spacings, and extrinsic phonon‐defects scattering. Ultimately, the anomalous anisotropy of phonon transport contributes to ≈100% increase in maximum dimensionless figure of merit compared to that attained in the direction parallel to pressing. This work demonstrates the efficacy of engineering wave‐like coherences for anisotropy decoupling of electrical and thermal transports to develop advanced quasi‐1D thermoelectric materials.