Diffusons and strong anharmonicity in BaCaX (X = Si, Ge) thermoelectric materials

Abstract

Zintl-phase compounds have garnered significant attention in the thermoelectric (TE) field due to their unique phonon-glass electron-crystal properties. In the current work, the crystal structure and TE properties of Zintl-phase BaCaX (X = Si, Ge) were theoretically explored using first-principles calculations, the two-channel model, and Boltzmann transport theory. The BaCaSi and BaCaGe compounds exhibit direct bandgaps of 0.79 eV and 0.80 eV at the level of Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional with a spin–orbital coupling effect. A steep dispersion of the conduction band minimum improves the electron mobility, while a flat dispersion at the valence band maximum improves the Seebeck coefficient, which are beneficial for enhancing the electronic transport. Mechanical and thermal stabilities of the BaCaX (X = Si, Ge) compounds are confirmed via elastic constant calculations and ab initio molecular dynamics simulations. Due to the complex crystal structures and weak chemical bonds, the BaCaX (X = Si, Ge) compounds exhibit significant anharmonicity, which leads to low lattice thermal conductivities. The rattling-like behavior of the Ba atom contributes to a substantial population of diffusons. The lone pair electrons and the rattling vibrations of the Ba atom play a pivotal role in suppressing phonon transport in BaCaX (X = Si, Ge). Combined with various carrier scattering mechanisms, the optimal figure of merit (ZT) values of 0.8 and 1.2 are achieved for n-type BaCaSi and BaCaGe compounds at 600 K, respectively, underscoring the great prospects of BaCaX (X = Si, Ge) compounds for high-performance TE applications. Our present work not only reveals the fundamental insights into electronic and thermal transport in BaCaX (X = Si, Ge) compounds but also provides theoretical guidance in the rational design of advanced thermoelectric materials.