Constructing Deep-Ultraviolet Nonlinear Optical Crystals via Synergistic Combination of Fluorinated Polyhedra and Polymerized BO3 Units

The development of modern science urgently needs the vital tool of deep-ultraviolet (DUV, λ < 200nm) all-solid-state lasers, while the exploration of DUV nonlinear optical (NLO) crystals is limited by their strict application conditions. Herein, we propose a structural design strategy for developing novel DUV NLO optical crystals through synergistic assembly of fluorinated polyhedral and planar B─O groups with balanced optical properties, namely, large second-harmonic generation (SHG) response, appropriate birefringence, and wide band gap. The key to achieve consistent alignment of π-conjugated functional groups lies in leveraging the “shearing” effect and directional polymerization capability of fluorinated polyhedra. Furthermore, based on this strategy, a series of alkali-metal fluorooxoborates M x (NH 4 ) 2- x B 8 O 12 F 2 (M = K, Rb; 0 < x < 2; KABF, RABF) and CsNH 4 B 8 O 12 F 2 (CABF) were successfully designed and synthesized. Driven by uniformly arranged [BO 3 F] 4− and chain-like aggregated [BO 3 ] 3− functional groups, these compounds achieve a well-balanced optimization of strong SHG response (1.6–1.7 × KDP, 1064–532 nm; 0.4–0.5 × BBO, 532–266 nm, respectively) and the shortest type-I phase-matching wavelength (161.5–168.6 nm). The inspiring results highlight their potential as candidates for DUV NLO crystals and provide support for the structure construction strategy using a synergistic combination of fluorinated polyhedra and polymerized BO 3 units.