Rattling vibration-induced low lattice thermal conductivity in Zintl-phase CaLiBi thermoelectrics

Thermoelectric (TE) materials have gained significant attention as crucial candidates for addressing the escalating challenges associated with environmental pollution and the global energy crisis. Utilizing first-principles calculations and Boltzmann transport theory, the crystal structure and the electronic and thermal transport properties of the CaLiBi compound are explored in the present work. The elastic modulus, ab initio molecular dynamics (AIMD) simulations, and phonon dispersion curves reveal the high mechanical, thermal, and dynamic stabilities of the CaLiBi compound. The CaLiBi compound is an indirect bandgap semiconductor with a bandgap of 0.73 eV in consideration of the Heyd-Scuseria-Ernzerhof (HSE06) functional and spin-orbit coupling (SOC) effect. The presence of multiple valleys in the electronic band structure enhances band degeneracy, which favors the high power factor. The rattling-like vibrations of Li atom produce large atomic displacement parameters, which weaken bonding interactions and induce lattice softening, thereby enhancing stronger anharmonicity. Meanwhile, the weak bonding between light Li and other atoms induces phonon localization, also intensifying both lattice anharmonicity and phonon scattering. Additionally, the flat phonon dispersion curves, especially at medium and high frequencies, reduce phonon group velocity, resulting in the suppression of phonon transport. These combined effects act cooperatively to markedly suppress phonon transport, ultimately resulting in a remarkably low lattice thermal conductivity. Considering multiple carrier scattering mechanisms and three-phonon anharmonic scattering, the CaLiBi compound exhibits promising prospects as an n-type TE material. Consequently, an optimal figure of merit (ZT) of 1.50 is achieved for the n-type CaLiBi compound at 600 K. This work not only offers fundamental insights into the thermal and electronic transport properties of the CaLiBi compound, but also sheds light on the theoretical design of Zintl phase TE materials.