Enhanced thermoelectric performance driven by the rattling effect and unique electronic structure in layered nitrides SrMN2 (M = Zr, Hf)

Layered nitrides have emerged as a new class of thermoelectric materials that have attracted considerable attention in recent years due to their unique geometric configurations and electronic structures. Based on first-principles calculations, this study combines the Boltzmann transport equation and self-consistent phonon (SCP) theory to systematically and comprehensively investigate the thermoelectric transport properties of the layered nitride SrMN${}_2$ (M = Zr, Hf). The results reveal that the significant rattling behavior of Sr atoms enhances the anharmonicity of the material, effectively reducing the lattice thermal conductivity. A comparative analysis between HA and SCP results confirms the necessity of incorporating higher-order anharmonicity and four-phonon (4ph) scattering mechanisms in strongly anharmonic systems. In addition, we comprehensively considered four major scattering mechanisms, namely MFP, ADP, IMP, and POP, to obtain more accurate electrical transport characteristics. The pronounced band dispersion and high degeneracy in the valence band endow the p-type SrMN${}_2$ with an excellent Seebeck coefficient and power factor at high temperatures. At 900K, the maximum ZT values of p-type SrZrN${}_2$ and p-type SrHfN${}_2$ reach 1.22 and 1.11, respectively, demonstrating outstanding thermoelectric performance. This study highlights that the strong anharmonicity arising from weakly bonded heavy elements, together with the unique two-dimensional electrical structure of SrMN${}_2$, plays a crucial role in enhancing thermoelectric performance, making it a highly promising candidate for high-performance thermoelectric applications.