An Unprecedented [BO2]‐Based Deep‐Ultraviolet Transparent Nonlinear Optical Crystal by Superhalogen Substitution

Solid‐state structures with the superhalogen [BO2]− have thus far only been observed with a few compounds whose syntheses require high reaction temperatures and complicated procedures, while their optical properties remain almost completely unexplored. Herein, we report a facile, energy‐efficient synthesis of the first [BO2]‐based deep‐ultraviolet (deep‐UV) transparent oxide K9[B4O5(OH)4]3(CO3)(BO2)·7H2O (KBCOB). Detailed structural characterization and analysis confirm that KBCOB possesses a rare four‐in‐one three‐dimensional quasi‐honeycomb framework, with three π‐conjugated anions ([BO2]−, [BO3]3−, and [CO3]2−) and one non‐π‐conjugated anion ([BO4]5−) in the one crystal. The evolution from the traditional halogenated nonlinear optical (NLO) analogues to KBCOB by superhalogen [BO2]− substitution confers deep‐UV transparency (< 190 nm), a large second‐harmonic generation response (1.0 × KH2PO4 @ 1064 nm), and a 15‐fold increase in birefringence. This study affords a new route to the facile synthesis of functional [BO2]‐based oxides, paving the way for the development of next‐generation high‐performing deep‐UV NLO materials.