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 (κl) 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 κl values of Zn3Te2, Cd3Te2, and Hg3Te2 are 0.732, 0.648, and 1.065 W m–1 K–1 at 300 K, respectively. These κl 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 κl 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 κl 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 κl.