Mechanical and electronic response of monolayer chromium trihalides CrX₃ (X = Cl, Br, I) under uniaxial strain

Recent advances in two-dimensional (2D) magnetic materials have promoted significant progress in low-dimensional magnetism and its technological applications. Among them, atomically thin chromium trihalides (CrX₃ with X = Cl, Br, and I) are among the most studied 2D magnets due to their unique magnetic properties. In this work, we employ density functional theory calculations to investigate the mechanical and electronic properties of CrX₃ monolayers in the presence of in-plane uniaxial strain. We calculate the strain-dependent energetics, stress-strain relationships, electron localization functions, and electronic density of states for two distinct strain directions: zigzag and armchair. Our results show that the ferromagnetic phase remains the magnetic ground state over a wide range of strain in both zigzag and armchair directions. The mechanical response exhibits a distinct anisotropy between these two directions, as reflected by the maximum value in the stress-strain relationship. As the halogen atom X becomes heavier, the maximum stress decreases from 6.04 (6.40) GPa to 3.20 (3.33) GPa in the zigzag (armchair) direction. By analyzing the electron localization function, we show the strain-induced variations in bonding characteristics and their impact on the mechanical strength of the materials. We also observe a strain-induced energy splitting of the Cr-d and X-p orbitals in the electronic density of states, and a strain-induced band-gap transition (from indirect to direct) in the electronic band structure, leading to a reduction of the electronic band gap with a directional dependence on the applied strain. Our results provide fundamental insights into the effects of uniaxial strain on the mechanical and electronic properties of CrX₃ monolayers, which are relevant for their potential use in strain-engineered electronics and spintronics.