Anharmonic rattling vibrations and multivalley bands in enabling Zintl-phase YbZn2X2 (X = As, Sb) thermoelectrics

Abstract

The design of high-performance thermoelectric (TE) materials requires the delicate balancing of inherently competing electronic and thermal transport properties. In this work, a comprehensive theoretical investigation of the Zintl-phase YbZn₂X₂ (X = As, Sb) materials is explored through first-principles calculations and Boltzmann transport theory, which simultaneously achieves a favorable confluence of high electronic transport efficiency and intrinsically low lattice thermal conductivity. The YbZn₂X₂ (X = As, Sb) materials possess direct band gaps of 1.23 eV and 0.39 eV using Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional, respectively. Weak crystal field effects suppress the splitting of valence bands, which enhance band degeneracy. The conduction bands exhibit multi-valley characteristics, promoting high carrier mobility and power factor. Moreover, flat phonon dispersion curves reduce phonon group velocities of YbZn₂X₂ (X = As, Sb) materials, and “rattling-like” vibrations of Zn atom and asymmetric Zn-X (X = As, Sb) bonds induce pronounced lattice anharmonicity. These characteristics effectively suppress thermal transport, which reduce the lattice thermal conductivities of YbZn₂As₂ and YbZn₂Sb₂ to 1.59 W/mK and 0.94 W/mK at 700 K, respectively. Consequently, the n-type doping YbZn₂As₂ and YbZn₂Sb₂ materials achieve the dimensionless figure of merits (ZT) of 0.62 and 0.75 @ 700 K, respectively, showing a synergistic optimization of the electronic structure and phonon scattering mechanisms. Notably, the predicted thermoelectric performance of YbZn₂Sb₂ is in excellent agreement with experimental measurements, affirming the robustness of the computational framework. These findings not only shed light on the fundamental coupling between electronic and lattice dynamics in Zintl-phase YbZn₂As₂ and YbZn₂Sb₂ materials, but also provide a predictive design strategy for next-generation TE materials with high performance.