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 [B3O6] functional species with the aid of structure‐directing property of mer‐isomer [YO3F3] octahedra, a new ultraviolet (UV) NLO material, Cs2YB3O6F2 with aligned arrangement of coplanar [B3O6] 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 [B3O6] groups. Remarkably, benefiting from the enhanced bonding force among functional units [B3O6], 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 mm3, indicating that it was a promising UV NLO crystal. This work provides a powerful strategy to design UV NLO materials with good performances.