[121] Exploring Ultralow Lattice Thermal Conductivity in a 2D Hexagonal Structure of M₃Te₂ (M = Zn, Cd, and Hg)

The quest for two-dimensional (2D) materials with intrinsically low lattice thermal conductivity (κ_l) has attracted significant attention due to their potential for enhancing the efficiency of a thermoelectric device. Herein, we present a hexagonal phase (space group P321) in 2D M₃Te₂ (M = Zn/Cd/Hg), discovered through a reliable ground-state search using an evolutionary algorithm combined with density functional theory (DFT) calculations. These semiconducting structures exhibit outstanding properties, including high mechanical flexibility (low stiffness), auxetic mechanical characteristics (negative Poisson’s ratio), and ultralow lattice thermal conductivity, which merit reporting. The lattice thermal transport parameters are determined through the iterative solution of the phonon Boltzmann transport equation (PBTE), leveraging inputs from DFT. Specifically, the κ_l values of Zn₃Te₂, Cd₃Te₂, and Hg₃Te₂ are 0.732, 0.648, and 1.065 W m⁻¹ K⁻¹ at 300 K, respectively. These κ_l values are significantly lower than those of many reported 2D materials such as SnSe₂ and SnSe, which is beneficial for thermoelectric applications. A comprehensive analysis of the underlying parameters reveals that the lower κ_l in M₃Te₂ structures is primarily due to their smaller phonon group velocities compared with other 2D materials. Interestingly, although Zn₃Te₂ exhibits a slightly higher group velocity than Cd₃Te₂ and Hg₃Te₂, its κ_l is lower than that of Hg₃Te₂. This discrepancy is attributed to Zn₃Te₂’s shorter phonon lifetime and larger mode-Grüneisen parameter (indicating stronger anharmonicity), which enhance phonon scattering and suppress thermal transport, thereby reducing κ_l.