Assembly of π‐Conjugated [B₃O₆] Units by Mer‐Isomer [YO₃F₃] Octahedra to Design a UV Nonlinear Optical Material, Cs₂YB₃O₆F₂

Achieving the extreme balance of the key performance requirements is the crucial to breakthrough the application bottleneck for nonlinear optical (NLO) materials. Herein, by assembly of the π‐conjugated [B 3 O 6 ] functional species with the aid of structure‐directing property of mer ‐isomer [YO 3 F 3 ] octahedra, a new ultraviolet (UV) NLO material, Cs 2 YB 3 O 6 F 2 with aligned arrangement of coplanar [B 3 O 6 ] groups has been synthesized. The polar material exhibits the rare coexistence of the largest second harmonic generation response of 5.6×KDP, the largest birefringence of 0.091 at 532 nm, the shortest Type I phase‐matching down to 200.5 nm and deep‐ultraviolet transparency among reported acentric rare‐earth borates with [B 3 O 6 ] groups. Remarkably, benefiting from the enhanced bonding force among functional units [B 3 O 6 ], a firm three‐dimensional framework is constructed, which facilitates the growth of large crystals. This can be proved by a block shape crystal with dimensional of 6×5×4 mm 3 , indicating that it was a promising UV NLO crystal. This work provides a powerful strategy to design UV NLO materials with good performances.