Covalent Interlayer Engineering Enables Ideal Birefringence and Strong SHG in Beryllium‐Free Deep‐UV NLO Borates

The development of deep‐ultraviolet (UV) nonlinear optical (NLO) crystals has long been hindered by an intrinsic trade‐off: planar π‐conjugated groups, when aligned in parallel to maximize second‐harmonic generation (SHG), inevitably produce excessive birefringence (e.g., β ‐BaB 2 O 4 , β ‐BBO), whereas non‐parallel arrangements suppress birefringence at the cost of SHG efficiency (e.g., LiB 3 O 5 , LBO). Here we break this deadlock by introducing covalent interlayer engineering into layered borates featuring parallel [B 3 O 6 ] units. Unlike conventional alkali/alkaline‐earth ions that act as passive spacers, rare‐earth cations (Sc 3+ , Lu 3+ ) form relatively strong covalent bonds with oxygen atoms from adjacent [B 3 O 6 ] layers, effectively competing for electron density and reducing the in‐plane polarizability anisotropy of the π‐conjugated rings. This strategy preserves the strong SHG activity inherited from β ‐BBO while fine‐tuning birefringence into the optimal range (0.077–0.081@1064 nm). The resulting beryllium‐free crystals, KCs 2 ScB 6 O 12 (KCSBO) and KCs 2 LuB 6 O 12 (KCLBO), exhibit deep‐UV transparency below 190 nm, phase‐matchable SHG efficiencies of 4.1–4.2×KDP, and ideal birefringence. Our work establishes covalent interlayer modulation as a new dimension for decoupling SHG and birefringence, offering a general paradigm for high‐performance deep‐UV NLO materials.