LiB2O3F: A Beryllium-Free Deep-Ultraviolet Nonlinear Optical Material Designed Based on a Boron-Rich Strategy

Exploring practical deep-ultraviolet (DUV) nonlinear optical (NLO) materials is still urgently required and severely challenging. Starting from an ideal hexagonal (BO)∞ layered geometry, we adopted a boron-rich strategy to introduce a high proportion of second harmonic generation (SHG)-active B–O(F) groups to design potential DUV NLO materials. Five novel noncentrosymmetric 3D crystal structures based on the LiB2O3F (LBF) system were predicted by employing the evolutionary algorithm combined with the density functional theory method. Benefiting from the high proportion of B–O(F) active groups and the favorable arrangements, the polar LBF-I and III crystals exhibit optimal comprehensive performances and surpass notable KBe2BO3F2 (KBBF) in all metrics: the SHG coefficient d11 is 0.88 pm/V for I (0.74 pm/V for III), being nearly 2 times that of KBBF (0.47 pm/V), and their wider band gaps together with larger birefringence lead to the shorter phase-matching wavelength (154.0 nm for I and 155.8 nm for III) compared to KBBF (161 nm). Significantly, the layering growth habit, the most serious drawback of KBBF, can be effectively improved in LBF owing to the direct connection between neighboring layers. Therefore, the LBF crystals would be promising high-performance DUV NLO materials. The in-depth mechanism explorations strongly demonstrate that the B-rich strategy is effective in enhancing the SHG effects and birefringence of DUV NLO crystals, meeting the original intention of our material design.