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

Abstract Solid‐state structures with the superhalogen [BO 2 ] − 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 [BO 2 ]‐based deep‐ultraviolet (deep‐UV) transparent oxide K 9 [B 4 O 5 (OH) 4 ] 3 (CO 3 )(BO 2 ) ⋅ 7H 2 O (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 ([BO 2 ] − , [BO 3 ] 3− , and [CO 3 ] 2− ) and one non‐π‐conjugated anion ([BO 4 ] 5− ) in the one crystal. The evolution from the traditional halogenated nonlinear optical (NLO) analogues to KBCOB by superhalogen [BO 2 ] − substitution confers deep‐UV transparency (<190 nm), a large second‐harmonic generation response (1.0×KH 2 PO 4 @ 1064 nm), and a 15‐fold increase in birefringence. This study affords a new route to the facile synthesis of functional [BO 2 ]‐based oxides, paving the way for the development of next‐generation high‐performing deep‐UV NLO materials.