Swarm-intelligent predictions of high-T_C polymorphs in monolayer CrI₃ above 77 K

Monolayer CrI₃, crystalizing in the $P\overline{3}1m$ space group, is a prototypical two-dimensional (2D) material for observing intrinsic ferromagnetic order. However, its relatively low Curie temperature (T_C) of 45 K severely limits its practical applications, highlighting the need to explore novel metastable polymorphs with enhanced magnetic properties. In this study, we employ a global crystal structure search technique combined with first-principles calculations to systematically investigate new monolayer CrI₃ phases. Our structural predictions identify two novel polymorphs with Cm and P2/m space groups, both of which are dynamically stable and exhibit significantly higher T_C values of 145 K and 81 K, respectively. Electronic property calculations show that the Cm phase is a half-metal, while the P2/m phase is semiconducting with a bandgap of 0.14 eV. Monte Carlo simulations attribute these enhanced T_C values to a notable increase in exchange interactions. These findings expand the known phase space of CrI₃ and provide a promising pathway for designing high-temperature 2D ferromagnets for next-generation spintronic applications.