Intrinsically Low Lattice Thermal Conductivity and Anisotropic Thermoelectric Performance in In‐doped GeSb₂Te₄ Single Crystals

Abstract Layer‐structured GeSb₂Te₄ is a promising thermoelectric candidate, while its anisotropy of thermal and electrical transport properties is still not clear. In this study, Ge_1–xIn_xSb₂Te₄ single crystals are grown by Bridgman method, and their anisotropic thermoelectric properties are systematically investigated. Lower electrical conductivity and higher Seebeck coefficient are observed in the c‐axis due to the higher effective mass in this direction. Intrinsically low lattice thermal conductivity is also observed in the c‐axis due to the weak chemical bonding and the strong lattice anharmonicity proved by density functional theory calculation. Indium doping introduces an impurity band in the bandgap of GeSb₂Te₄ and leads to the locally distorted density of states near the Fermi level, which contributes to enhanced Seebeck coefficient and improved power factor. Ultimately, a peak zT value of 1 at 673 K and an average zT value of 0.68 within 323–773 K are obtained in Ge_0.93In_0.07Sb₂Te₄ along the c‐axis direction, which are 54% and 79% higher than that of the pristine GeSb₂Te₄ single crystal, respectively. This study clarified the origin of intrinsic low lattice thermal conductivity and anisotropy transport properties in GeSb₂Te₄, and shed light on the performance optimization of other layered thermoelectric materials.