Influence of anharmonicity on the thermoelectric properties of alkali antimonide compounds M3Sb (M = Na, K, Rb, Cs)

This study systematically explores the mechanical, transport, and thermoelectric properties of the alkali antimonide compounds M3Sb (M = Na, K, Rb, Cs) using first-principles calculations and the Boltzmann transport equation. We found that considering quartic anharmonic effects leads to temperature-dependent phonons undergoing a frequency shift and overall phonon spectrum hardening, thereby increasing the phonon group velocity. Additionally, this study reveals that alkali-metal atoms play an important role in the phonon frequency of M3Sb (M = Na, K, Rb, Cs). The quartic anharmonicity strengthens as the mass of the alkali-metal atom increases. Moreover, as the atomic number of alkali-metal atoms increases, the interaction between atoms weakens, resulting in a gradual decrease in bulk modulus (B), shear modulus (G), and Young's modulus (E). We also observe that, beyond 400 K, the lattice thermal conductivity kappa(L) of Rb3Sb is lower than that of Cs3Sb, which is mainly attributed to the larger scattering rate of Rb3Sb. By considering various scattering mechanisms, we obtain reasonable electron relaxation times and electron transport properties. Because of the large electronic dispersion band at the conduction-band minimum, p-type M3Sb exhibits high electron mobility. Finally, owing to the low lattice kappa(L) and good power factor, these four p-type alkali antimonide compounds demonstrate excellent thermoelectric performance.