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

Abstract 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 [BF 4 ] − unit with a large HOMO–LUMO band gap was introduced to the Na−B−O−F system and Na 4 B 8 O 9 F 10 was designed and synthesized successfully for the first time. Na 4 B 8 O 9 F 10 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, Na 4 B 8 O 9 F 10 is the first acentric crystal with [BF 4 ] − units among the reported metal–fluorooxoborate systems, involving isolated [BF 4 ] − and novel [B 7 O 10 F 6 ] 5− fundamental building blocks.