Boosting Ultraviolet Phase‐Matching Performance of B ₃ O ₇ ‐Based Systems via Synergistic Fluorination and Organic Modification Strategy

ABSTRACT Ultraviolet (UV) nonlinear optical (NLO) crystals with excellent performance are indispensable for advancing high‐precision laser technologies. Anionic groups directly govern the key optical properties including second‐harmonic generation (SHG) response, birefringence, and phase‐matching capability. The [B 3 O 7 ] 5− unit is a classic NLO‐active building block, but its inherent structural limitations (e.g., non‐uniform arrangement in parent crystals) often lead to insufficient birefringence and restricted short‐wavelength phase‐matching potential. Herein, we proposed a synergistic fluorination and organic modification strategy to tailor the [B 3 O 7 ] 5− group, successfully designing and synthesizing two novel magnesium fluoroborate malonate compounds: Mg(BF 2 C 3 O 4 H 2 ) 2 (H 2 O) 2 (MgBCOFH‐1) and Mg(BF 2 C 3 O 4 H 2 ) 2 C 3 O 4 H 4 (H 2 O) 2 (MgBCOFH‐2). Both compounds feature an unprecedented [BF 2 C 3 O 4 H 2 ] − building unit derived from the structural evolution of [B 3 O 7 ] 5− . Notably, MgBCOFH‐1 and MgBCOFH‐2 exhibit enhanced birefringence values of 0.100 and 0.073 @ 546 nm, respectively, significantly surpassing that of the parent [B 3 O 7 ]‐based LiB 3 O 5 (LBO, 0.042 @ 546 nm). More importantly, MgBCOFH‐2 achieves a phase‐matching wavelength of 223 nm (a 54 nm blue‐shift relative to LBO) alongside a moderate SHG intensity (∼1 × KDP @ 1064 nm), making it a promising candidate for a fourth‐harmonic UV laser generation. This work establishes a rational structural modification paradigm for upgrading conventional NLO‐active building blocks and provides insights into the development of high‐performance short‐wavelength UV NLO crystals.