Ultralow Lattice Thermal Conductivity in Two-Dimensional Zintl CaZn₂Sb₂: Phonon Softening, Anharmonicity and Four-phonon Scattering

Abstract

Zintl compounds have attracted increasing interest in thermoelectric applications because their bonding complexity and structural softness can favor intrinsically low lattice thermal conductivity. Here, the electronic band structure, mechanical properties, lattice dynamics, and intrinsic lattice thermal transport of 2D CaZn2Sb2 are investigated using first-principles calculations and the linearized phonon Boltzmann transport equation, intrinsic three-phonon and isotope scattering. The 2D CaZn2Sb2 is found to be mechanically and dynamically stable. The directly calculated lattice thermal conductivity decreases from 0.2138 W m⁻¹ K⁻¹ at 300 K to 0.096 W m⁻¹ K⁻¹ at 700 K. An additional RTA-based assessment shows that the inclusion of four-phonon scattering reduces the lattice thermal conductivity by 42.4% at 300 K and 54.5% at 700 K, highlighting the increasing importance of higher-order anharmonic scattering at elevated temperatures. After correcting the vacuum-dependent normalization using an effective thickness derived from the out-of-plane repeat length of the parent bulk unit cell, the corresponding three-phonon LBTE values remain ultralow at 0.6414 and 0.288 W m⁻¹ K⁻¹, respectively. The suppressed thermal transport is associated with pronounced acoustic-phonon softening, a low Debye temperature of 92.66 K compared with 224 K for bulk CaZn2Sb2, large low-frequency mode Grüneisen parameters, limited phonon group velocities, and short phonon lifetimes. Within the present particle-like LBTE framework, the optical branches collectively contribute approximately 65.94% of the calculated lattice thermal conductivity at 300 K, whereas the three acoustic branches account for approximately 34.07%. When normalized by the number of branches, the average contributions are approximately 5.50% per optical branch and 11.36% per acoustic branch, indicating that the larger total optical contribution mainly reflects the cumulative contribution of the twelve optical branches rather than a higher per-branch transport efficiency. Total thermal analysis shows that 90% of the lattice thermal conductivity is accumulated below 6.02 THz and within phonon mean free paths shorter than 22.35 nm. Independent Slack-model estimates decrease from 0.2383 W m⁻¹ K⁻¹ at 300 K to 0.1021 W m⁻¹ K⁻¹ at 700 K and reproduce the same temperature dependence as the first-principles results. This qualitative consistency indicates that the ultralow lattice thermal conductivity is associated primarily with the low characteristic phonon energy scale, heavy constituent atoms, acoustic-mode anharmonicity, and restricted phonon transport, while the collective contribution of optical modes remains substantial.