Constructing Deep‐Ultraviolet Nonlinear Optical Crystals via Synergistic Combination of Fluorinated Polyhedra and Polymerized BO ₃ Units

ABSTRACT 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.