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 K-9[B4O5(OH)(4)](3)(CO3)(BO2) & sdot; 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 pi-conjugated anions ([BO2](-), [BO3](3-), and [CO3](2-)) and one non-pi-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 (1.0xKH(2)PO(4) @ 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.