Strain-Induced Ferroelectricity and Second-Harmonic Generation in a TaOI₂ Monolayer

Recently, two-dimensional (2D) layered niobium oxide dihalides NbOX₂ (X = Cl, Br, I) have attracted significant interest due to their unique ferroelectric and second-harmonic generation (SHG) properties. However, 2D TaOI₂ materials, possessing a similar structure to NbOX₂, show no ferroelectric polarization and SHG due to the existence of inversion symmetry. Our first-principles calculations show that a nonpolar to ferroelectric phase transition is found under tensile strain along the Ta-O direction, which induces strong ferroelectric polarization and SHG response in the TaOI₂ monolayer. Moreover, exciton effects play a crucial role in linear absorbance and SHG response, which also can be modified to a large extent by strain. Furthermore, the competitive relationships between ferroelectric polarization (P_s) and optical gap E₀ in determining SHG coefficients are revealed by a volcano plot between the SHG coefficients and combined parameter β₁ = P_s·E₀. Our work has important implications for developing 2D ferroelectric and nonlinear optical materials from existing inversion-symmetric materials.