Unveiling Exceptional Thermoelectric Performance in Non-Layered X₂S₂ (X = K, Rb) via Coexistence of the Flat Band and High Dispersion: Ultralow Lattice Thermal Conductivity and High Anisotropic ZT Values

The mechanical, thermal transport, and thermoelectric characteristics of X2S2 (X = K, Rb) compounds are comprehensively investigated with first-principles calculations, merging self-consistent phonon theory, bubble theory, and Boltzmann transport equations. Our results reveal that the X2S2 materials exhibit good mechanical and thermal stability. Importantly, strong lattice anharmonicity is found in X2S2. In addition to considering the quartic anharmonic renormalization of phonon frequency, we also introduce the bubble term (which represents cubic anharmonic renormalization), thereby leading to an obvious phonon frequency shift. Thus, the accurate lattice thermal conductivity (κL) can be obtained by the SCP + Bubble-3ph4ph method, considering the temperature-driven phonon frequency shift and scattering processes involving three-phonon (3ph) and four-phonon (4ph) scatterings. The calculated κL is found to have ultralow values of 0.84 (0.56) and 0.78 (0.49) W K–1 m–1 along the a(b)- and c-axes at 300 K for K2S2 (Rb2S2), respectively. Moreover, the coexistence of high electronic dispersion band edges and electronic flat band edges along the Γ–A direction causes the large power factor. Consequently, a high figure of merit (ZT) is achieved along the c-axis. The outstanding ZT peak values of n-type K2S2 (2.62) and n-type Rb2S2 (2.11) exceed 2 along the c-axis at 900 K, indicating their great potential for thermoelectric application. Remarkably, the ZT values display significant anisotropy. These results emphatically reveal that the nonlayered bulk n-type K2S2 material holds great promise for the development of thermoelectric devices with anisotropic properties in the future.