Metal-free borates have emerged as a new class of solid-state chemistry and related crystalline materials with high performance. However, the discovery of metal-free borate crystals that can meet the phase-matching behavior in the short-wavelength ultraviolet (UV) spectral region is extremely limited. Against this background, two new guanidinium fluorooxoborates with the chemical formula of [C­(NH2)3]­[B3O3F2(OH)2] and [C­(NH2)3]2[B3O3F4(OH)] have been discovered and characterized as high-performance candidates for short-wavelength UV nonlinear optical (NLO) applications. The optimally aligned configuration of coplanar [C­(NH2)3] cations and [B3O3F2(OH)2]/[B3O3F4(OH)] heteroanionic units makes both crystals have a sufficiently large NLO coefficient, birefringence, and band gap, which indicate that both metal-free hydroxyfluorooxoborates are promising NLO crystals. The well-ordered configurations of OH/F anions in two borates were identified using various approaches, and the origin of high optical performance was validated by additional measurements and calculations. Our findings verified the feasibility of searching NLO crystals in the short-wavelength region in a metal-free borate system via an eco-friendly and low-cost way.

Polarization modulation of deep-UV light is of significance to current technologies, and to this end, the birefringent crystal has emerged as an invaluable material as it allows for effective light modulation. Herein, a double-modification strategy driven by F and OH anions that makes double effects towards the critical property enhancement of deep-UV birefringent crystals is proposed. This leads to a new hydroxyborate (NH 4 ) 4 -[B 12 O 16 F 4 (OH) 4 ] with giant cluster as a deep-UV birefringent crystal with large birefringence (Δn exp. =

A new metal-free monofluorophosphate with a large birefringence has been reported. The reported birefringence sets a record among fluorophosphates and it originates from the preferential arrangement of non-π-conjugated [HPO3F] units.