Na4B8O9F10: A Deep‐Ultraviolet Transparent Nonlinear Optical Fluorooxoborate with Unexpected Short Phase‐Matching Wavelength Induced by Optimized Chromatic Dispersion

Exploring significant ultraviolet/deep-ultraviolet nonlinear optical (NLO) materials is hindered by rigorous and contradictory requirements, especially, possessing a moderate optical birefringence to meet phase-matching (PM). Except for suitable birefringence, small chromatic dispersion is also crucial to blue-shift the PM wavelength. Here, the introduction of a fluorinated tetrahedral boron-centred chromophore strategy was proposed to optimize the chromatic dispersion. Herein, the [BF4 ]- unit with a large HOMO-LUMO band gap was introduced to the Na-B-O-F system and Na4 B8 O9 F10 was designed and synthesized successfully for the first time. Na4 B8 O9 F10 with an optimized chromatic dispersion can achieve a short second harmonic generation PM wavelength of 240 nm with a relatively small birefringence (0.036@1064 nm). Notably, Na4 B8 O9 F10 is the first acentric crystal with [BF4 ]- units among the reported metal-fluorooxoborate systems, involving isolated [BF4 ]- and novel [B7 O10 F6 ]5- fundamental building blocks.