not-yet-known not-yet-known not-yet-known unknown The mechanical, thermal transport and thermoelectric characteristics of X2S2 (X = K, Rb) compounds are comprehensively investigated with the 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, the 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 scatterings processes involving three-phonon (3ph) and four-phonon (4ph) scatterings. The calculated κL is found to be ultralow values with 0.84 (0.56) and 0.78 (0.49) WK-1m-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 Γ to 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 the significant anisotropy. These results emphatically reveal that the non-layered bulk n-type K2S2 material holds great promise for the development of thermoelectric devices with anisotropic properties in the future.

We employ advanced first-principles methodology, merging self-consistent phonon theory and the Boltzmann transport equation, to comprehensively explore the thermal transport and thermoelectric properties of KCdAs. Notably, the study accounts for the impact of quartic anharmonicity on phonon group velocities in the pursuit of lattice thermal conductivity and investigates 3ph and 4ph scattering processes on phonon lifetimes. Through various methodologies, including examining atomic vibrational modes and analyzing 3ph and 4ph scattering processes, the paper unveils microphysical mechanisms contributing to the low $\kappa_{L}$ within KCdAs. Key features include significant anisotropy in Cd atoms, pronounced anharmonicity in K atoms, and relative vibrations in non-equivalent As atomic layers. Cd atoms, situated between As layers, exhibit rattling modes and strong lattice anharmonicity, contributing to the observed low $\kappa_{L}$. Remarkably flat bands near the valence band maximum translate into high PF, aligning with ultra-low $\kappa_{L}$ for exceptional thermoelectric performance. Under optimal temperature and carrier concentration doping, outstanding \textit{ZT} values are achieved: 4.25 (a(b)-axis, p-type), 0.90 (c-axis, p-type), 1.78 (a(b)-axis, n-type), and 2.36 (c-axis, n-type).