First-principles study on the anisotropic physical properties of the layered nitride BaMN<sub>2</sub> (M = Ti, Zr, Hf)
Authors: Yu Jian-Xiang  , Liang Hua-Lin  , Yang Yi-Jun  , Ming Xing  ,   1 College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China;,   2 College of Physics and Electronic Information Engineering, Guilin University of Technology, Guilin 541004, China
Published: February 2025

Ternary layered nitrides have garnered widespread attention due to their unique electrical, optical and optoelectronic properties, which are promising for the fabrication of low-cost and highefficiency optoelectronic materials, solar cell materials and photocatalysts. Although there are no experimental reports on BaTiN<sub>2</sub> to date, BaZrN<sub>2</sub> and BaHfN<sub>2</sub> have been synthesized experimentally by solid state method. However, their optical and electrical transport properties have not been systematically investigated. The purpose of this paper is to systematically investigates the mechanical, electronic, optical absorption, carrier transport, and dielectric response properties of BaMN<sub>2</sub> (M = Ti, Zr, Hf) nitrides by first-principles calculations based on density functional theory. Due to the quasi-two-dimensional layered arrangement of [MN<sub>2</sub>]<sup>2-</sup> slabs, the ionic bonds between Ba<sup>2+</sup> and N<sup>3-</sup>, and the weak interactions between the slabs, deformation along this direction is most likely to occur under the action of external stress. BaMN<sub>2</sub> nitrides exhibit significant anisotropic physical properties. Firstly, the mechanical properties of BaMN<sub>2</sub>, such as bulk modulus, shear modulus, Young's modulus and Poisson's ratio, show prominent anisotropy. The lower modulus, higher Poisson's ratios and Pugh's modulus ratios indicate good flexibility of the BaMN<sub>2</sub> nitrides. In addition, BaMN<sub>2</sub> has indirect bandgap values (1.75-2.25 eV) within the visible-light energy range, which meets the basic requirement for the band gap of a photocatalyst for water splitting (greater than 1.23 eV). Moreover, BaMN<sub>2</sub> has suitable band-edge positions. The appropriate bandgap values and band-edge positions indicate their broad application prospects in the absorber layer of solar cells and photocatalytic water decomposition. Attributed to the pronounced differences in the effective mass of its charge carriers in different directions, BaMN<sub>2</sub> exhibit ultrahigh anisotropic carrier mobilities (on the order of 10<sup>3</sup> cm<sup>2</sup>s<sup>-1</sup>v<sup>-1</sup>) and lower exciton binding energies. At the same time, there are significant differences in atomic arrangement and bonding interactions along the in-plane and out of plane directions, resulting in high anisotropic visible-light absorption coefficients (on the order of 10<sup>5</sup> cm<sup>-1</sup>) in the low energy regions. In contrast, the opportunities for electrons to transition from occupied to unoccupied states increase, leading to more complex light absorption and relatively reduced anisotropy in higher energy regions. Furthermore, the special layered structure has lower polarizability and higher vibration frequency along the vertical direction perpendicular to the [MN<sub>2</sub>]<sup>2-</sup> layers, rendering BaMN<sub>2</sub> nitrides show high dielectric constants. These excellent anisotropic mechanical, optoelectronic, and transport properties allow BaMN<sub>2</sub> layered nitrides to be used as promising semiconductor materials in the fields of optoelectronics, photovoltaics, and photocatalysis.