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.