Guanidinium Fluorooxoborates as Efficient Metal-free Short-Wavelength Nonlinear Optical Crystals

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.