Anomalous Mass‐Trend Lattice Thermal Conductivity and Exceptional Thermoelectric Performance in AMTe2 (A = Na, K, Rb, Cs; M = Sc, Y) Driven by Hierarchical Rattling

We conduct a systematic first‐principles study of the hierarchical ( = Na, K, Rb, Cs; = Sc, Y), focusing on their structural stability, lattice dynamics, and thermoelectric transport properties. All compounds exhibit excellent mechanical, dynamical, and thermodynamic stability, indicating their good potential for experimental synthesis. A salient result is an anomalous mass–thermal conductivity relation in , where the lattice thermal conductivity is markedly lower than that of despite the lighter Na atomic mass. This anomaly originates from the localized rattling modes of ions, which induce pronounced anharmonic phonon scattering and strong phonon localization. The electronic structure displays high band degeneracy characteristics near the Fermi level, enabling simultaneous optimization of the Seebeck coefficient and electrical conductivity. As a result, peak values of all materials exceed 2.6 at 900 K, with n‐type NaScTe 2 reaching a record of 4.2 along the ‐axis due to its ultralow thermal conductivity and optimized power factor. These findings establish compounds as prime candidates for high‐temperature, high‐efficiency thermoelectric energy conversion.